Support sub-document operations within transactions

Right now only 4 methods are available : get, insert, replace, remove.

This means that in order to mutate documents, you have to fetch the entire document to get its cas, tx links and tx metadata, then you have to push the entire document over the network again.

When you only want to push an element into an array, that's quite sub-optimal.

Also, if you want to check if a document exists, once more you have to fetch the entire document. If projections were possible, this could be avoided.