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.
I made it os.Stderr because I didn't want it to be the treated as the Gearman response for the command. I'm not sure what we want there? I was thinking no response for now? Thoughts?

I should certainly comment it better either way.