postgresql: (Auto)tune DB pool sizes based on max_connections

Created by: keegancsmith

We have had Sourcegraph.com and customer issues due to opening too many connections to postgresql. We should consider to either:

1. Autotune our db pool sizes. We can query max_connections.
2. Autotune max_connections (probably can't due to different sized envs)
3. Adjust DB pool size to be much more conservative.

Customers:

• https://app.hubspot.com/contacts/2762526/company/419771425