[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 
Hmm. The `o` field in Toku is the full object pre-image right?

I wonder if it makes sense to just emit that full hash. You can certainly imagine cases where that would be useful to have access to, and something you might want to code your application against if you knew you were pure-Toku. It seems sad to lose that information in the interests of unified interface, since a generic application will only look at `op['o']['_id']` anyway.