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 (merged)

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.