feat(mastodon): Database prepare statements

Cherry-Pick of https://patch-diff.githubusercontent.com/raw/mastodon/chart/pull/5.patch



[PATCH] Add the PREPARED_STATEMENTS environment variable and set it
 to its default value Being able to control it can be useful in situations
 where prepared statements don't work as expected. Once such case can be
 pgbouncer as provided by DigitalOcean for connection pooling.

Co-authored-by: Cees-Jan Kiewiet <ceesjank@gmail.com>