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 ?
I don't think this will give you the value you're looking for though.

What's going to happen, I think is that the `value` key will be the same for every single `locktype`. Since you're not getting the `count(*)` per type, it's the total count of locks total.

So just at a glance, this isn't going to give you the right data.