Many of our test cases only use a single NamedInstance from the
`servers` fixture. Introduce `nsX` helper fixtures to simplify these
tests and reduce boilterplate code further.
Specifically, the test no longer has to either define its own variable
to extract a single server from the list, or use the longer
servers["nsX"] syntax. While this may seem minor, the amount of times it
is repeated across the tests justifies the change. It also promotes
using more explicit server identification, i.e. `nsX`, rather than
generic `server`. This also improves the clarity of the tests and may be
helpful in traceback during debugging as well.
The new 'tcp-primaries-timeout' configuration option works the same way
as the existing '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. The default is 15 seconds.
Also, while accommodating zone.c's code to support the new option, make
a light refactoring with the way UDP timeouts are calculated by using
definitions instead of hardcoded values.
