Log when a slurper is started and when the monitor finishes
Use a shared MongoClient for all rivers that use the same server(s)

The new MongoClientService is a per-node service in elasticsearch, and provides access
to the MongoClient instance(s) needed by the rivers.
Remove some unused imports
Use MongoDB 2.6.5 for testing
Mark the MongoClientService as @Singleton

While there: add some logging when it is started/stopped
Add a Module that explicitly creates the MongoClientService