Add sampling to unsampled event logging

Has issues:
- needs dependency injection
- needs to be DRY
- should not load mw.eventLogging when we are not in sample

Due to urgency this will be fixed in another commit.

Change-Id: I0df067a619109a7c945f82c8d33fa2e621217f0b
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/619
Cherry-picked from commit 171b13149e1fb6edabff533aa8d3b93bb520452a,
with some manual editing.
Conflicts:
	resources/mmv/mmv.DurationLogger.js