LLThread::currentID() used to return a U32, a distinct unsigned value
incremented by explicitly constructing LLThread or by calling LLThread::
registerThreadID() early in a thread launched by other means. The latter
imposed an unobvious requirement on new code based on std::thread. Using
std::thread::id instead delegates to the compiler/library the problem of
distinguishing threads launched by any means.

Change lots of explicit U32 declarations. Introduce LLThread::id_t typedef to
avoid having to run around fixing uses again if we later revisit this decision.

LLMutex, which stores an LLThread::id_t, wants a distinguished value meaning
NO_THREAD, and had an enum with that name. But as std::thread::id promises
that the default-constructed value is distinct from every valid value,
NO_THREAD becomes unnecessary and goes away.

Because LLMutex now stores LLThread::id_t instead of U32, make llmutex.h
#include "llthread.h" instead of the other way around. This makes LLMutex an
incomplete type within llthread.h, so move LLThread::lockData() and
unlockData() to the .cpp file. Similarly, remove llrefcount.h's #include
"llmutex.h" to break circularity; instead forward-declare LLMutex.

It turns out that a number of source files assumed that #include "llthread.h"
would get the definition for LLMutex. Sprinkle #include "llmutex.h" as needed.

In the SAFE_SSL code in llcorehttp/httpcommon.cpp, there's an ssl_thread_id()
callback that returns an unsigned long to the SSL library. When LLThread::
currentID() was U32, we could simply return that. But std::thread::id is very
deliberately opaque, and can't be reinterpret_cast to unsigned long.
Fortunately it can be hashed because std::hash is specialized with that type.

Id rather than sha1 hash, since that is rarely used in modern
certs. The previous form was storing trusted certs using an empty sha1
hash value as the key, which meant most certificates matched... not good.

Modify the LLCertException to pass certificate information back as
LLSD rather than an LLPointer<LLCertificate>, because when the
exception is being thown from the certificate constructor that results
in one of a couple of other exceptions (even refcounting won't save
you when the problem is that the thing you're pointing to never
finished coming into being properly).

Update the certificates in the llsechandler_basic_test to modern
conventions, and extend the classes to allow for an optional
validation date so that the test can use a fixed date. Also make all
the certificates include the plain text form for ease of reference.

(transplanted from 8b0c42b1a4f0416a17c8ec6078a85c5773f69a25)

don't set stopped flag until recorder object has been deleted

fix for crash on exit resulting from 8c0e024d0c33

over to get_thread_recorder() to be agnostic about which thread we're running on

replace llinfos, lldebugs, etc with new LL_INFOS(), LL_DEBUGS(), etc.

Have the ::notifyLoadedMeshes() method doing correct locking
and stall avoidance at the same time.  This method now does
lazy mutex lock acquisition (trylock()) and if it fails on
either, it gives up and comes back later.  Capture the maximum
number of sequential failures and report this at the end of
the run in the log.  (So far, with big mesh regions, I've
only seen 1s and 2s.)  Locking/mutex requirements sorted in
other locations as well.  LLMutex gets trylock() method as
well as new LLMutexTrylock scoped locking class.  Clean up
some documentation, more to do.

fixed ostream precision munging in llsys

various fixes to lltrace
start() on started recording no longer resets
fixed various instances of unit forgetfullness in lltrace
recording split now has gapless timing
scene monitor now guarantees min sample time
renamed a bunch of stats
added names to debug thread view on windows

also, removed LLTrace::init and cleanup
removed derived class implementation of memory stat for LLMemTrackable
is automatic now

removed LLThreadLocalSingleton
collapsed all thread recorder classes to single type, LLTrace::ThreadRecorder
moved fasttimer stack head to llthreadlocalsingletonpointer via ThreadRecorder

pulled swap() out of ui time block
cleaned up internal lltrace dependencies, factored out common accumulator definitions

fixed copy construction behavior of Recordings to not zero out data
split measurement into event and sample, with sample representing
a continuous function

changed thread id declaration to be local to llthread.cpp and use
currentID() uniformly across platforms

fixes to merge

fixed units conversion so that trace getters return convertable units
removed circular dependencies from lltrace*
converted more stats to lltrace

Fix non-thread-safe refcounting of LLHTTPClient::Responder and fix out-of-order deletion of LLTextureFetch on shutdown

renamed LLTrace::ThreadTrace to LLTrace::ThreadRecorder
renamed LLTrace::Sampler to LLTrace::Recording

cleaned up API
samplers are now value types with copy-on-write buffers under the hood
removed coupling with LLThread