fix: usr: Stop reusing outgoing TCP connections the peer has already closed

``named`` could hand a new query an idle forwarder/upstream TCP or TLS
connection that the peer had already closed, causing the query to fail
(and CLOSE-WAIT sockets to pile up). Idle reused connections are now
watched, so a close is noticed and the connection is dropped instead of
reused. A new ``tcp-reuse-timeout`` option controls how long an idle
outgoing connection is kept open for reuse (default 5 seconds).

Closes #6171

Merge branch '6171-tcp-reuse-idle-read' into 'main'

See merge request isc-projects/bind9!12289