[9.20] fix: usr: Limit the outgoing UDP send queue size

If the operating system UDP queue gets full and the outgoing UDP sending
starts to be delayed, BIND 9 could exhibit memory spikes as it tries to
enqueue all the outgoing UDP messages.  Try a bit harder to deliver the
outgoing UDP messages synchronously and if that fails, drop the outgoing
DNS message that would get queued up and then timeout on the client side.

Closes #4930

Backport of MR !9506

Merge branch 'backport-4930-limit-the-UDP-send-queue-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!9511