lib: Slight Optimization of thread handling.

lib: Slight Optimization of thread handling.

This commit does these things:

1) Make thread_add_unuse own the setting of THREAD_UNUSED.
2) Move thread->hist finding to to thread_get.
   We are storing the thread->hist even when the thread
   is on the unused.  This means that we check to see
   if the funcname or func have changed and we get new
   history.  Else we've probably just retrieved the last
   unused which has the same func/funcanme.  This is
   a common practice to do THREAD_OFF/THREAD_ON in
   quick succession.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com.