Recycle stale busy connection

[mostafa]

Similar to #99 (#101).
Authenticated connections are the trickiest to replace.

• Investigate connection timeout mechanism in PostgreSQL to see if recycling connections are feasible or practical #118
• Add connection activity metrics
• Monitor connection activity metrics
• Act on metrics

[mostafa]

Should be done after #57 and https://github.com/gatewayd-io/gatewayd-plugin-proposals/issues/2.

[mostafa]

Connections are now properly and gracefully closed (as fixed in #343), and all the connection handling logic works as expected so far.