While investigating NODE-1326, I am finding a lot of issues related to how the SessionPool tracks sessions.

If we can assume that a topology only ever belongs to one MongoClient at a time, and that the topology gets created/destroyed alongside the MongoClient, then I think we should implement the following:

1. Track all active and available ServerSessions, and all active ClientSessions, from within the SessionPool. Currently, we track all active ClientSessions from the MongoClient, and all available ServerSessions inside the SessionPool.
2. Ensure that the topology is responsible for sending an endSession for all ServerSessions when the topology is closed.

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