Split caching database from data source

[davenquinn]

There is no special reason that the PostgreSQL tile cache needs to be in the same database as the primary Macrostrat database. We should make it configurable to a separate database, which will help with load balancing. We could also consider using a separate database connection for cache reads and writes for extra performance.

[davenquinn]

This should be designed so that the POSTGRES_DB configuration variable is used by default, but can be specialized (e.g., with CACHE_READ_DB and CACHE_DB connection strings). I'm not quite sure how best to configure this to work with the Postgres Kubernetes operator.

This should also be extended to the CLI, so that the caching tables are created in the appropriate database.