[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 don't think we need to deal with this now, but one thing we'll need to watch for is that the Toku oplog has more than just `$set` and `$unset` ops - it'll represent `$inc` and such directly. At some point, we might want to translate those into the `$set` and `$unset`s that Mongo would generate, but that probably doesn't need to happen now.