JGroups subsystem parser should not allow thread pool to be defined per protocol

In this case, the parser does not match the schemas. There are 3 issues here:

1. Thread pool elements should only be parsed for schema version >= 3.0
2. Thread pool elements are only valid children of a <transport/> element, but are currently accepted as children of a <protocol/> element.
3. We should always create thread pool resources, even if xml is undefined - that way the thread pool parameters, as applied to the runtime, are expressed in the model.