sss_client: Use unique lock for memory cache
    
    Previously the sma lock was used as for communication with
    responder. However it would cause a deadlock in case of
    re-checking memcache after acquiring the lock and before communication with
    responder..
    
    Required by:
    https://fedorahosted.org/sssd/ticket/2581
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>