Add sendSizeAndMsgWithTimeout to send size and data in a single call

[rgacogne]

Short description

sendSizeAndMsgWithTimeout() groups the sending of the size as a uint16_t in network byte order and the corresponding data in a single sendmsg() syscall, handling short writes and timeouts if needed. It also supports optional source address and interface number. This PR makes use of this new function in the Remote Logger (protobuf) and dnsdist TCP workers.
It also adds TCP Fast Open support toward backends in dnsdist, if the MSG_FASTOPEN flag is available.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled and tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added regression tests
• added unit tests