rx: Add rx_NewThreadId function

rx: Add rx_NewThreadId function

The fileserver and the fsync server were locking an internal RX
mutex, and incrementing an internal counter in order to obtain fake
pthread thread IDs. Instead of letting them muck around in the
internals of RX, provide an API that can be called to obtain a
ThreadId counter, and use that API throughout the code.

Change-Id: I68f41d1486cdafeb757da2c0df899ae1ca2b2bfc
Reviewed-on: http://gerrit.openafs.org/2859
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>