journal-max-io cannot be set to 1 with ASYNCIO journal

https://issues.apache.org/jira/browse/ARTEMIS-2928 has introduced https://github.com/JCTools/JCTools multi-producer multi-consumer queues that has the strong requirements of > 1 capacity: they throw an exception if such requirement isn't met.

This can be fixed by using 2 as the minimum queue capacity, without affecting the factory behavior ie don't need to be a precise capacity matching maxIO, but at least enough to contains maxIO callbacks.