Ticket 47781 - Server deadlock if online import started while
     server is under load
    
    Bug Description:  Online import of ldif containing repl data deadlocks server.
    
    Fix Description:  Replace the replica lock(PRLock) with a PRMonitor reentrant lock.
    
    https://fedorahosted.org/389/ticket/47781
    
    jenkins: passed
    
    Reviewed by: nhosoi(Thanks!)
    
    (cherry picked from commit 0e11f7182a3b8f42f90ec8335936965b88a11336)