[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 (closed)
Backport of MR !9506 (merged)