Reworked thread code to better support thread-library specific

r/w locks and thread pools.  Hide internal structures (using
pthread'ish technics).  Place common code in threads.c.  Move
no-thread code to thr_stub.c.  Move thread pool code to tpool.c.
Removed setconcurrency call from initializer, added 'concurrency'
directive to slapd.  Tested code under pthreads, pth, and no-threads.