postgres: split locks by type

A table lock is probably more problematic than a row lock, so lets
not lump them together.

Note: this is currently not even tested. Just putting it here as a request for comment. I think this would be a good idea personally as currently you can't really get much useful info out of the lock stats) but as it is would break compatibility. Any comments @kormoc or @mattrobenolt ?
yeah of course. If I'd have even run this before making the PR, I'd have noticed that ;) Thanks