Add connection pool settings

[johnbeynon]

This hopefully resolves the issues experienced by customers in #inc-2022-06-03-strapi-database-connection-terminates-unexpectedly

It typically manifested as a Server unhealthy message with this in the logs:

May 31 11:12:01 AM [2022-05-31 10:12:01.823] debug: ⛔️ Server wasn't able to start properly.

May 31 11:12:01 AM [2022-05-31 10:12:01.824] error: select "t0"."id" from "admin_users" as "t0" limit $1 - Connection terminated unexpectedly

May 31 11:12:01 AM error Command failed with exit code 1.

May 31 11:12:01 AM Error: select "t0"."id" from "admin_users" as "t0" limit $1 - Connection terminated unexpectedly at Connection.<anonymous>

https://render.zendesk.com/agent/tickets/17938
strapi/strapi#11860

[johnbeynon]

Also validated by comment here - strapi/strapi#11860 (comment)