[9.18] fix: usr: Fix nsupdate hang when processing a large update

To mitigate DNS flood attacks over a single TCP connection, we throttle the connection when the other side does not read the data. Throttling should only occur on server-side sockets, but erroneously also happened for nsupdate, which acts as a client. When nsupdate started throttling the connection, it never attempts to read again. This has been fixed.

Closes #4910

Backport of MR !9709

Merge branch 'backport-4910-nsupdate-hangs-when-processing-large-update-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!9835