Add oplog-dump cmd

This is a simple command to dump all the oplog data after a certain
timestamp.

I haven't written any tests because it's not easy to test the things
related to the oplog, but if anyone has suggestions I would love to hear
them.
Very good question... I want to say somewhere else (it seems weird for it to live in this repo just because it uses this command), but I don't have a better suggestion - it sounds small enough that giving it its own repo feels silly. So maybe it is just best to live here.