Add dual-writes and read-your-own-writes libs

[ezekg]

First pass. Commit history is a mess, but I'll clean it up later (probably). Introduces a new Clickhouse database, which we'll eventually use as the primary source for logs and analytics, and soon, meter consumption ingest.

Introduces a new gem called dual_writes that replicates writes from Postgres to Clickhouse, which we're going to be using for request and event logs. Planning on running dual-writes for a few weeks — enough to collect a sufficient backlog in order to switch over analytical queries to Clickhouse without breaking everybody's charts. After the cutover, we can backfill historical log data for customers with longer retention periods than the default. Right now, the gem only has a Clickhouse strategy, but additional strategies can be implemented to e.g. replicate from Postgres to MySQL.

In addition, we're also introducing a new gem called read_your_own_writes, or ryow, which lays the groundwork for a read replica database with a non-session-based "read-your-own-writes" resolver — since our API doesn't rely on sessions for most API requests (except Portal), clients are identified via a combination of account, session, auth, and IP.

Both of these gems will eventually be open sourced when time allows.