new: usr: Implement tcp-primaries-timeout

The new `tcp-primaries-timeout` configuration option works the same way
as the older `tcp-initial-timeout` option, but applies only to the TCP
connections made to the primary servers, so that the timeout value can
be set separately for them. By default, it's set to 150, which is 15
seconds.

Closes #3649

Merge branch '3649-configurable-xfr-tcp-timeouts' into 'main'

See merge request isc-projects/bind9!9376