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.