[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.
we could also hash the stream content of the binary fields. its only in the stdClass representation when loading a node that you have length instead of content. but lets move this to its own issue - we need to figure out if it needs to be customizable or pluggable in some way, and if jackrabbit already does something about it.