ISPN-4706 Don't abort Marshalling on interruption

The issue was identified via stress tests; I would hit it regularly and everything works much smoother with this fix.
The StressTest itself is not ready for merging yet.

 - https://issues.jboss.org/browse/ISPN-4706

I think removing this check might cause JBoss Marshalling to throw bogus exceptions if the node really is shutting down and the writers map has been cleared. It would have been better to replace the interruption check with `if (!started)`.