dnsdist: Add recvmmsg support for backend responses

[rgacogne]

Short description

Needs testing, especially performance testing, documentation and perhaps even a switch in the configuration.

Closes #16625

Checklist

I have:

• read the CONTRIBUTING.md document
• read and accepted the Developer Certificate of Origin document, including the AI Policy, and added a "Signed-off-by" to my commits
• 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)

[miodvallat]

The bot complains that recvData[idx] is a function call in disguise. The following ought to silence it:

Suggested change

	fillMSGHdr(&msgVec[idx].msg_hdr, &recvData[idx].iov, &recvData[idx].cbuf, sizeof(recvData[idx].cbuf), reinterpret_cast<char*>(recvData[idx].packet.data()), initialBufferSize + 1, &recvData[idx].remote);
	fillMSGHdr(&msgVec[idx].msg_hdr, &recvData[idx].iov, &recvData[idx].cbuf, sizeof(cmsgbuf_aligned /*recvData[idx].cbuf*/), reinterpret_cast<char*>(recvData[idx].packet.data()), initialBufferSize + 1, &recvData[idx].remote);

[omoerbeek]

Maybe use decltype?

[miodvallat]

Ah yes, that ought to work.