services/horizon/expingest: Separate DB connection pool for ingestion service

[bartekn]

Currently, all of the Horizon services share the same DB connection pool. In case of increased number of HTTP requests, it's possible that ingestion service will have to wait for an idle connection. We need to create a separate connection pool (with a number of open connections equal to [horizon max] - [max needed by ingestion at any given time: 2 right now?]) for ingestion service to make sure it can always proceed.

[ire-and-curses]

Now that we have split ingesting and front-facing nodes, this isn't necessary for our cluster. It could still potentially be helpful for single machine deployments. Let's revisit if that seems to be the case.

[bartekn]

Reopening. Frontend nodes still run ingestion because they ingest offers into in-memory order book.