dnsdist: Limit the number of concurrent console and web connections

[rgacogne]

Short description

Closes #4978.

This PR also cleans up a bit the console connection code, wrapping the socket to make sure it doesn't leak (existing code looked good but required some bookkeeping that the new one doesn't) and removing a couple unneeded allocations + copies.

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)

[rgacogne]

Rebased to fix a conflict, added some unit tests in the process.