Remove TODO in tailable cursor example.

(I was wrong: adding a 'ts' query parameter won't activate the oplog's code
path on the server.)
Dead code. (Obviated by Motor pool rewrite.)
Unused import.
MotorGreenletEvent wakes waiters in FIFO order.
Test MotorGreenletEvent.
Obsolete guard when importing pymongo.auth.
Refactor cleanup code for auth tests.