Evaluate transaction handling / clickhouse-sink-connector

[bmtcril]

We should evaluate whether https://github.com/Altinity/clickhouse-sink-connector is a good replacement for our MySQL based sinks. It may be a more robust solution and get around a number of issues with the current sinks & Celery saturation / transaction handling.

If it seems like it won't be a good choice for us, we should at least convert the remaining MySQL sinks to use the on_commit handler so that we can work around issues related to rollbacks, trying to read uncommitted data, etc. See this #108 for more information.