Clickhouse (#66)

* Get stats from clickhosue

* Pull stats from clickhouse

* Use correct Query namespace

* Use Clickhouse in unit tests

* Use Clickhouse in stats controller tests

* Use fixtures for unit tests

* Add Clickhouse to travis

* Use Clickhouse session store for sessions

* Add garbage collection to session store

* Reload session state from Clickhouse on server restart

* Query from sessions table

* Trap exits in event write buffer

* Run hydration without starting the whole app

* Make session length 30 minutes

* Revert changes to fingerprint schema

* Remove clickhouse from fingerprint sessions

* Flush buffers before shutdown

* Use old stats when merging

* Remove old session schema

* Fix tests with CH sessions

* Add has_pageviews? to Stats

* Use CH in staging

* Update schema

* Fix test setup