added tokumx check by cloning and modifying mongo

- fixed calculation of replication lag (and in mongo check)
- added opcountersRepl (and in mongo check)
- added tokumx-specific metrics present in tokumx 1.4.2
- added some more replication info
- added some sharding info (could use more in the future)
- changed to MongoClient to keep up with modern pymongo usage
- added db/collection/index stats with tags