Issue 50636 - Crash during sasl bind
    
    Bug Description:
    	Sasl bind registers IO layers (sasl_IoMethods) that will be
    	pushed (and called) by the next incoming operation.
    	So the next incoming operation should synchronize itself
    	with the sasl bind.
    
    Fix Description:
    	The call to connection_call_io_layer_callbacks, that pushes
    	registered methods, must hold c_mutex so that it let
    	a pending sasl bind to fully register the methods.
    
    https://pagure.io/389-ds-base/issue/50636
    
    Reviewed by: Ludwig Krispenz, Mark Reynolds