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/aithreadsafe.h

@@ -673,18 +673,18 @@ protected:
 
 #ifdef LL_DEBUG
 	mutable bool mAccessed;
-	mutable apr_os_thread_t mTheadID;
+	mutable AIThreadID mTheadID;
 
 	void accessed(void) const
 	{
 	  if (!mAccessed)
 	  {
 		mAccessed = true;
-		mTheadID = apr_os_thread_current();
+		mTheadID.reset();
 	  }
 	  else
 	  {
-		llassert_always(apr_os_thread_equal(mTheadID, apr_os_thread_current()));
+		llassert_always(mTheadID.equals_current_thread());
 	  }
 	}
 #endif