Use static for cache dir and cache extension to eliminate mutex lock overhead to generate cache file path

This changeset hooks up many things that have been in progress and moves things about between llfilesystem and lldiskcache - there is still some bookkeeping work left but this is the first version that appears to work and actively manage the cache

Complete the change from lldiskcache -> llfilesystem and then addition of new lldiskcache implementation

Rename lldiskcache.* to llfilesystem.* - i think this is the right name since it's responsible for performing file system operations and (will eventually) delegrate to a separate disk cache component to save/load data and keep track of metadata etc.

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.

Effective rename of DRTVWR-506-simple since there is no way to rename a branch. Cloned canonical viewer into DRTVWR-519, copied over the files that changed from DRTVWR-506-simple and pushed back.  Once I am satisfied everything is correct, DRTVWR-506-simple will be removed

Checker: UNINIT_CTOR
Function: LLVFile::LLVFile(LLVFS *, const LLUUID &, LLAssetType::EType, int)
File: /indra/llvfs/llvfile.cpp

dead field.

Result of svn merge -r107256:107258 svn+ssh://svn/svn/user/phoenix/license_2009_merge into trunk. QAR-1165

QAR-369

QAR-369 - viewer-cleanup2-7 81916 merged into release.

Result of svn merge -r71162:71205 svn+ssh://svn/svn/linden/branches/new-license into release. only changes files which are not deployed or the comments section of code.