[INPROGRESS] First version of tokumx converter

This pull request aims to add tokumx support to mongoriver by converting tokumx oplog records to mongo oplog records.

cc @ebroder 
Could have a `last_oplog_entry` helper or something. (I think you probably still want to grab the `{_id: -1}` sort even with toku. A "latest_timestamp" is always going to be an approximation with Toku regardless because transactions)