Mongo client service

Move the management of the MongoClient instances into a per-node service, rather than having each river manage its own instance.

This avoids using too many connections with hundreds of rivers.

See also https://github.com/richardwilly98/elasticsearch-river-mongodb/pull/371#issuecomment-62022068 for a more detailed rationale.
you have a client here that's not coming from the MongoClientService. seems like you should use mongoClientServiceEverywhere?