Implement multi-use snapshots - #3615

Multi-use snapshots trigger an "implicit" server-side transaction, and capture its ID on the first request. Subsequent requests return that ID, allowing for isolation from other changes. We default to multi_use=False because that mode is much more performant for the simple case.

This feature is one which we originally decided to leave out, but the P0 system test list requires that it be implemented.

I think that a commitwise review might be easier than reviewing the whole enchilada.