Changed locking for function calls into _ldap

HI!

After Howard's answer

http://www.openldap.org/lists/openldap-software/201005/msg00054.html

I've relaxed the locking when calling functions of _ldap. Solely
_ldap.initialize() is now protected by a lock.

Also there's now debug logging for the locks if trace_level is 2 or higher.

Please review and test the code in HEAD especially with OpenLDAP 2.3 libs and
in heavily threaded environment.

Ciao, Michael.