Add support for read replicas when using MySQL or PostgreSQL

[akajla09]

Is your feature request related to a problem? Please describe.
The Warrant service currently supports MySQL, PostgreSQL and SQLite as databases. Even though the service itself is horizontally scalable (i.e. can add more service nodes), it is limited by the underlying dbs. A common HA deployment pattern for these single-master dbs is to deploy multiple read replicas (or readers) to take load off of the main writer node, especially in read-heavy deployments as is commonly the case for Warrant.

As such, the Warrant service should be able to make use of a multi-node db cluster (multiple readers, single writer) to enable even higher scale for check requests.

Describe the solution you'd like

• Add the ability to maintain distinct reader and writer db connection pools.
• Usage of 'zookie' (Add support for 'zookie' protocol as described in Zanzibar spec #161) will account for any replication lag between writer and its reader instances.

Additional context

• Note that this solution is only for single-master dbs. Multi-master dbs (like Google Spanner) provide their own consistency guarantees.