Add Admin Only Port for Emergencies

[reltuk]

When max_connections is reached, there is no mechanism for a DBA to login and start killing sessions. We could open an additional SQL port which only allows privileged user access. Similar to:

https://dev.mysql.com/doc/refman/8.4/en/administrative-connection-interface.html

Related to #8942.

[macneale4]

If we're blocking, we should just block until a new connection becomes available.

dolthub/vitess#404

That's the only low hanging fruit of this issue. I think if we want to get into rejecting connections, that's going to need some config.

For the last issue - an additional admin port - That would be great. We should do that, but it's considerably more work.

[macneale4]

I've decided to add the back_log and max-connections-timeout options because they hanging behavior is pretty horrible for any production application.

[macneale4]

Partially implemented with: dolthub/vitess#406

Updating description to be about admin port.