driver.KillCursors being called by changestream's Next does not close the connection it takes a lease on which can cause non deterministic hands in Disconnect.

Used the following to detect this in a test: https://gist.github.com/edaniels/3968df00d4ed5cb614756f89d8efb09a

Old:
I'm hitting a pretty odd issue trying to run tests in Stitch on latest driver code. During test teardown every once in a while we hang indefinitely trying to disconnect acquiring a semaphore.

See these stack dumps:
https://evergreen.mongodb.com/test_log/5c17c1512fbabee87f89f729#L5729
https://evergreen.mongodb.com/test_log/5c17d52f0305b956eee01a4d#L2224