MessageDrivenComponent startDelivery/stopDelivery is not thread safe

WFLY-4470 made a change to prevent MDBs being activated or deactivated multiple times, but it is not thread safe. A volatile boolean is used for the flag, but there is no protection against multiple threads invoking the methods simultaneously.

Possible effects include:
activating the endpoint twice, with (probably) only one deactivation later, leading to not de-registering XA resources from the recovery manager properly
activate() and deactivate() running in the wrong order if done by separate threads