Improve max_lifetime handling

[mirek26]

Does your PR solve an issue?

Implements proposal (1) + (2) from #3051

1. Move max lifetime checking from acquire to release. This reduces the number of query-time reconnections due to max lifetime
2. Improves the maintenance task to handle connection replacement if min_connections > 0. This especially improves the scenario where the application is alive for a significant amount of time before it starts getting traffic. Without this change, sqlx opens min_connections connections and they just sit idle. Then query time, we discover they are over max_lifetime and have to reconnect. With this change, the connections are actually replaced every max_lifetime and can be used when needed.

I'm open to other ideas but this seems to me like a strictly better default than the current one. One question is whether we should be replacing connection below min_connections due to being idle - this PR does that but I can see the argument for doing that only with max_lifetime