DB_THREAD and Concurrent DB support

["Kurt D. Zeilenga" <Kurt@OpenLDAP.org>]

I made changes which effectively serialize access to the
BerkeleyDB API as more work is needed.

If someone has time, they could:

If HAVE_BERKELEY_DB_THREAD, use a giant reader/writer
lock instead of giant mutex.

If HAVE_BERKELEY_CDB, use Concurrent DB provided
synchronization, e.g. no giant mutex (or r/w lock).

Please coordinate on this list to avoid duplicate effort.
Please adhere to devel/contribution guidelines, etc.
	http://www.openldap.org/devel/

Kurt