Bug 745259 - Incorrect entryUSN index under high load
                 in replicated environment
    
    https://bugzilla.redhat.com/show_bug.cgi?id=745259
    
    Description:
    . Changed the backend entry lock from PRLock to PRMonitor to
      allow re-entrant locking.  In ldbm_back_delete and ldbm_
      back_modify, backend entry lock was released before calling
      be_preop plugins and re-acquired just after that to avoid
      the deadlock by the be_preop plugins called from the same
      thread. The short no-lock window was the cause of the entry-
      usn corruption.  By replacing PRLock with the re-entrant
      PRMonitor, we could eliminate the no-lock window.
    . Cleaned up compiler warnings.