[WIP] Moving node validation from JL-Doctrine-DBAL to Jackalope

As suggested in the comments for this code in the doctrine-dbal implementation, this PR will aim to move the node validation to here.

The validate method currently also adds additional node operations (e.g. for node autocreate) to the transport, so I think it should probably be renamed / refactored.
hah, so not all history is lost :-D but i have no idea anymore what i was thinking about. probably cycling through everything. i suggest we just remove that comment. i don't see how we could be much more efficient.