[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 
I'd probably be a bit less user-friendly here. It's much better, in my mind, to have this throw immediately when you start the tailer because you passed the wrong kind of timestamp, then to have it confusingly start to tail from the beginning of the oplog.

I think I'd write this by first conditionalizing on `database_type`, then asserting that the `from` is appropriate for that type (and raising if not)