Remove in-memory boolean to indicate config.server.sessions collection set up

Problem Statement

config.system.sessions is not automatically recreated after a drop in 4.0.

Proposed Solution

1. Create a method in SessionsCollection named onSessionsCollectionDropped. This method would be a no-op everywhere except for the config server child class, where we would flip the _collectionSetUp boolean to false.
2. Expose the SessionsCollection to outside callers through a new LogicalSessionCache method. If we decide this isn't alright, we can just mirror the sessions collection with onSessionsCollectionDropped.
3. Hook into the config server opObserver onCollectionDropped to get the SessionsCollection and call onSessionsCollectionDropped.
4. Create a ticket to consider making the LogicalSessionCache or SessionsCollection its own opObserver.

Rejected Solutions

1. Create an opObserver for the SessionsCollection. We think that this is overkill for the current bugfix.