Switch from "Monotime" to "monotonic"

See this discussion:

https://github.com/tornadoweb/tornado/pull/1579

For a monotonic clock in Python 2, the "monotonic" package is marginally superior to "Monotime". We should update our documentation to recommend monotonic instead of Monotime, and update pymongo/monotonic.py to try "import monotime" before "import monotonic".