Prevent crash caused by race condition during plugin initialization.
    
    Processing of global configuration was postponed. Now the persistent
    search watcher thread doesn't change configuration directly.
    
    The problem was that isc_task_beginexclusive()
    was called before task associated with psearch watcher thread
    was fully initialized.
    
    Now the watcher enqueues a new configuration change event
    and this event is processed later by another thread.
    
    https://fedorahosted.org/bind-dyndb-ldap/ticket/110
    
    Signed-off-by: Petr Spacek <pspacek@redhat.com>