dnsdist: Fix TCP short writes handling

[rgacogne]

Short description

Fix a bug in the handling of short writes over TCP. It could have led to sending data from the beginning of the buffer twice instead of from the proper place in the buffer, sending corrupted responses.
Since it requires filling out the TCP window, and thus large answers toward the same TCP client to occur, AXFR answers are the most likely to be affected.
Introduced in #4985, so only master is affected.

Fixes #5494.

Checklist

I have:

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