Spelling
Fix MongoReplicaSetClient race conditions PYTHON-467

* RSState, Member, and MovingAverage are now immutable
* In refresh(), try up members before down ones
* A test_ha fixup (clear process-list after killing them in each test, so we don't re-kill previous tests' processes)
Correct a docstring
More debug output from ha_tools
test_ha cleanup
More reliable test_primary_stepdown
Remove test_fork's docstring