Depend on tx manager available rather than if inside immutant.

While it may be a lot of work to support running all of immutant with a
custom transaction manager, it seems doable for the messaging sub-system
and these small changes are intended to allow the messaging sub-system
to function as long as there is an injected XA manager available by
changing a few checks to be for an available manager instead of if being
run inside of immutant container.