Ticket 50636 - Crash during sasl bind
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.
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.
Reviewed by: Ludwig Krispenz, Mark Reynolds
Platforms tested: F28
Flag Day: no
Doc impact: no