Add AIThreadID - Cleanup of apr_os_thread* related code.

Apart from just really cleaning things up and moving
everything into one class regarding thread IDs (ie,
is_main_thread(), comparing ID's etc), this also
fixes an obscure bug where LL was casting thread ID's
to U32 and then compared those to find out if it
the same thread. It's theoretically possible that
such fails on a 64bit OS.

By generalizing the interface, I adopted the use
of a thread-local cache for the current thread ID
as used by LLMutex et al, so now all code benefits
from that. The idea was even extended to now also
be used for is_main_thread() tests and even resetting
a thread ID to the ID of the current thread.

indra/llcommon/CMakeLists.txt

@@ -15,8 +15,9 @@ include_directories(
     )
 
 set(llcommon_SOURCE_FILES
-	aiframetimer.cpp
-	imageids.cpp
+    aiframetimer.cpp
+    aithreadid.cpp
+    imageids.cpp
     indra_constants.cpp
     llallocator.cpp
     llallocator_heap_profile.cpp
@@ -24,7 +25,7 @@ set(llcommon_SOURCE_FILES
     llapr.cpp
     llaprpool.cpp
     llassettype.cpp
-	llavatarname.cpp
+    llavatarname.cpp
     llbase32.cpp
     llbase64.cpp
     llcommon.cpp
@@ -103,6 +104,7 @@ set(llcommon_HEADER_FILES
     CMakeLists.txt
 
     aiframetimer.h
+    aithreadid.h
     aithreadsafe.h
     bitpack.h
     ctype_workaround.h