Bug 725743 - Make memberOf use PRMonitor for it's operation lock
    
    We currently use a mutex for the operation lock in the memberOf
    plug-in.  This can cause a deadlock issue since the locks are not
    re-entrant.  When memberOf performs an internal operation that
    triggers another plug-in to perform an internal operation, a
    deadlock will occur if this second internal operation triggers
    memberOf again.
    
    This patch switches memberOf to use PRMonitor instead, which is
    re-entrant.