Reconnect gateway replica SSH tunnels after process exit

[peterschmidt85]

Fixes #4352.

When a replica's SSH process exits, nginx keeps forwarding to its abandoned Unix socket and returns HTTP 502 indefinitely.

• Supervise gateway SSH processes and reconnect on exit, reusing the same socket so nginx needs no reconfiguration.
• Retry failed reconnections with backoff and at most eight simultaneous attempts to avoid overloading the gateway during a shared outage.
• Clean up SSH and ProxyCommand processes during failed startup, replica removal, and cancellation.
• Use uvloop on gateways to avoid allocating one waiting thread per replica on Python 3.10.

Existing gateways receive the fix automatically when the server is updated to the fixed release under default settings.