PgCat holds more idle connections than PgBouncer

[zainkabani]

Describe the bug
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We've migrated several databases to PgCat from PgBouncer and found that with the same connection limits and timeouts configured PgCat ends up creating more connections

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Expected behavior
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Idle connections aren't great to have on the DB and it would be good to figure out why PgCat doesn't evict connections and/or requires more connections

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[levkk]

I think this is because bb8 asynchronously spawns new connections when multiple clients request one and the pool doesn't have any idle ones available: https://github.com/djc/bb8/blob/main/bb8/src/inner.rs#L102-L147. I believe Pgb is doing the same thing synchronously, but I don't recall.