fix(NODE-3263): force closing a pool closes active connections

[rose-m]

Prior to this PR using MongoClient.close(true) to force-close the client did also trigger a force-close of the connection pool and the individual connections. However, as the pool only tracked available connections (not all connections that were created by the pool), it could not close connections that were active.

This fix PR adds:

• Tracking all connections that are created by the pool in a new internal Set.
• On force-close instead of iterating only over the available connections it will iterate over all known connections and force-close them.
• A test case is provided to ensure that also checked-out connections are force-closed.

The behavior and semantics of the MongoClient.close() (i.e. without force = true) remain unchanged.

The test case will fail when the iteration in line 342 is made only over the available connections.

[dariakp]

Converting this PR to draft pending additional investigation of NODE-3263

[Xunnamius]

Just got bitten by this in my CLI app. I can't do a clean exit even after all operations have completed and after calling MongoClient.close(true); my program hangs unless I add a process.exit(0) at the end, which makes unit and integration testing a bit harder. Any plans on merging this fix?