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).

Unfortunately, for Unbound I can't see separate configurability...
so I didn't touch its config.
The separation for kresd is recent master:
knot-resolver!1026

contrib/job_manager/files/kresd.conf.j2

@@ -15,6 +15,10 @@ net.listen('::1', {{ tls_port }}, {tls = true})
 -- essential for TLS_FORWARD in Docker with lots of respdiff jobs
 net.tcp_pipeline(65535)
 
+-- avoid TC flags returned to respdiff
+local _, up_bs = net.bufsize()
+net.bufsize(4096, up_bs)
+
 {% if forward %}
 -- forwarding
 policy.add(policy.all(

contrib/job_manager/files/named.conf.j2

@@ -12,6 +12,8 @@ options {
 	allow-query     { localhost; };
 	max-cache-size	1048576000;
 
+	max-udp-size 4096; # reduce TC towards clients (respdiff)
+
 	deny-answer-addresses {
 		0.0.0.0/8;
 		::ffff:0.0.0.0/104;