contrib/job_manager: config servers' UDP size limit

The limits are going to change around Flag Day 2020, which could lead to
differences in TC returned to respdiff, so let's hardcode the limit
*towards clients* to 4096 (higher so we don't compare empty answers).
That will also keep it up for forwarding (for simplicity),
which is OK, I think.

The separation for kresd is recent master:
knot-resolver!1026

Docs:
 https://bind9.readthedocs.io/en/v9_16_6/reference.html?highlight=max-udp-size#tuning
 https://nlnetlabs.nl/documentation/unbound/unbound.conf/#max-udp-size
I'm not so sure about the meaning in Unbound; in both BIND and Unbound
the default hasn't been lowered yet, so *currently* it's a no-op there
and the point is to keep the client side this way for later releases.