Two sleep() methods: one accepting F32Milliseconds, or in general any LLUnits
time class; the other accepting any std::chrono::duration.

The significant thing about each of these sleep() methods, as opposed to any
freestanding sleep() function, is that it only sleeps until the app starts
shutdown. Moreover, it returns true if it slept for the whole specified
duration, false if it woke for app shutdown.

This is accomplished by making LLApp::sStatus be an LLScalarCond<EAppStatus>
instead of a plain EAppStatus enum, and by making setStatus() call set_all()
each time the value changes. Then each new sleep() method can call
wait_for_unequal(duration, APP_STATUS_RUNNING).

Introducing llcond.h into llapp.h triggered an #include circularity because
llthread.h #included llapp.h even though it didn't reference anything from it.
Removed. This, in turn, necessitated adding #include "llapp.h" to several .cpp
files that reference LLApp but had been depending on other header files to
drag in llapp.h.

This set of changes reverts the merge with master (git revert c83e740e) and results in a version of the DRTVWR-519 that matches what was presemt before it was deployed as a release viewer *plus* 3 small fixes from Maxim (See commits).  This branch can now be used for additional fixes before eventually being used to release D-519 as normal

This reverts commit e61f485a, reversing
changes made to 00c47d07.

The folder where the disk cache lives was originally renamed from llvfs to llcache but @henri's suggestion that that doesn't reflect the other files in the same place and it should be llfilesystem is a good one so I changed it over

Renamed the references to LLVFile and llvfile.* source code plus cmake scripts to use a different name - lldiskcache - since that more closely resembles what it is (or will be) now that the VFA is no more

First part of change to remove LLVFS from the Viewer. Consists of code changes to remove LLVFS and LLVFSThread classes along with the associated source files. The existing llvfs folder is renamed to llcache. Also includes changes to CMake script in many places to reflect changes. Eventually, llvfile source file and class will be renamed but that is not in this change.

Got into D503 by accident

In three different places we use the same pattern: an ssl_thread_id_callback()
function (a static member of LLCrashLogger, in that case) that used to be
passed to CRYPTO_set_id_callback() and therefore returned an unsigned long
representing the ID of the current thread.

But GetCurrentThread() is a HANDLE, an alias for a pointer, and you can't
uniquely cram a 64-bit pointer into an unsigned long.

Fortunately OpenSSL has a more modern API for retrieving thread ID. Pass
each ssl_thread_id_callback() function to CRYPTO_THREADID_set_callback()
instead, converting it to accept CRYPTO_THREADID* and call
CRYPTO_THREADID_set_pointer() or CRYPTO_THREADID_set_numeric() as appropriate().

The LLApp API used to consist of init(), mainLoop(), cleanup() methods. This
makes sense -- but on Mac that structure was being subverted. The method
called mainLoop() was in fact being called once per frame. There was
initialization code in the method, which (on Mac) needed to be skipped with an
already-initialized bool. There was a 'while' loop which (on Mac) needed to be
turned into an 'if' instead so the method would return after every frame.

Rename LLApp::mainLoop() to frame(). Propagate through subclasses LLAppViewer
and LLCrashLogger. Document the fact that frame() returns true to mean "done."
(This was always the case, but had to be inferred from the code.)

Rename the Mac Objective-C function mainLoop to oneFrame. Rename the C++ free
function it calls from runMainLoop() to pumpMainLoop(). Add comments to
llappdelegate-objc.mm explaining (inferred) control flow.

Change the Linux viewer main() and the Windows viewer WINMAIN() from a single
LLAppViewer::mainLoop() call to repeatedly call frame() until it returns true.

Move initialization code from the top of LLAppViewer::frame() to the init()
method, where it more properly belongs. Remove corresponding
mMainLoopInitialized flag (and all references) from LLAppViewer.

Remove 'while (! LLApp::isExiting())' (or on Mac, 'if (! LLApp::isExiting())')
from LLAppViewer::frame() -- thus unindenting the whole body of the 'while'
and causing many lines of apparent change. (Apologies to reviewers.)

There are four LLApp states: APP_STATUS_RUNNING, APP_STATUS_QUITTING,
APP_STATUS_STOPPED and APP_STATUS_ERROR. Change LLAppViewer::frame() return
value from (isExiting()) (QUITTING or ERROR) to (! isRunning()). I do not know
under what circumstances the state might transition to STOPPED during a
frame() call, but I'm quite sure that if it does, we don't want to call
frame() again. We only want a subsequent call if the state is RUNNING.

Also rename mainLoop() method in LLCrashLogger subclasses
LLCrashLoggerWindows, LLCrashLoggerMac, LLCrashLoggerLinux. Of course it's
completely up to the frame() method whether to yield control; none of those in
fact do. Honor protocol by returning true (frame() is done), even though each
one's main() caller ignores the return value.

In fact LLCrashLoggerWindows::mainLoop() wasn't using the return protocol
correctly anyway, returning wParam or 0 or 1 -- possibly because the return
protocol was never explicitly documented. It should always return true: "I'm
done, don't call me again."

MAINT-6322 fix merge error that prevented crash dumps from being located for upload (and add better logging)

MAINT-6322 fix merge error that prevented crash dumps from being located for upload (and add better logging)

Refactor any remaining LLCore::HTTPHandlers to use boost::shared_ptr
Started minor refactor in the materials manager into coroutines (unfinished)

Convert LLCore::HttpHeaders to use shared_ptr<> rather than an intrusive_ptr<> for refrence counting.