[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.
How hard is it to implement ETAG generation? Does it need to be implemented at the implementation level, or can we just generate a hash based on the sum of the binary fields? (which, I am led to believe, are always going to contain the *length* of the binary, not the binary itself?)