Fix for connection pooling problem

When using connectionsPerHost configuration and/or mongo:connection-pooling-profile, the current version of the mongo-connector create a new connection for every message processor execution.  This is because the Mongo Java Driver uses thread local storage to keep state for DB.requestStart(), DB.requestEnsureConnection(), and DB.requestDone().

This pull request solves the issue by doing the following:
* Remove the MongoClientImpl.openSession() method that forced all Mongo client calls to use requestStart() and requestEnsureConnection()
* Add new Processor methods that allow a user to start and end [consistent requests](http://docs.mongodb.org/ecosystem/drivers/java-concurrency/) if they need to