Add ExperimentCounter module

This is used to manipulate counters associates with experiments. This
commit adds the type and moves callers, but does not move
implementations.
Move implementations into ExperimentCounter
Counters to class not just module
Make redis an instance variable

This means connections will be restablished between requests
Add sentinel redis discovery support
ExperimentCounter does not delete experiment
Merge pull request #118 from mixcloud/ts-redis-sentinel

Add support for redis sentinel