dnsdist: Ponder using `recvmmsg` to receive UDP responses from backends

[rgacogne]

• Program: dnsdist
• Issue type: Feature request

Short description

We currently have an option to use recvmmsg to receive queries from clients, which is providing a nice performance boost. It would be nice to have an option to use it also when receiving responses from backends.
Note that I'm not mentioning sendmmsg on purpose, because:

• the performance gain is not as obvious
• it requires changing a lot of code to queue the packets to send