dnsdist: Improve TCP connection reuse, add metrics

[rgacogne]

Short description

This PR:

• adds metrics about TCP connection reuse (total number of connections opened and reused per backend) ;
• adds a new command to dump the states of TCP connections (incoming and outgoing) for debugging purposes ;
• reuses the most recently used connection first, and for connections that have not been used in the last second checks whether it has been closed in the meantime. Detecting such a closed connection earlier make it possible to reuse another queued connection right away ;
• provides better messages (only seen at verbose level) when a TLS connection has been closed by the other end.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)