Add stale-cache-enable option and disable serve-stable by default

Add stale-cache-enable option and disable serve-stable by default

The current serve-stale implementation in BIND 9 stores all received
records in the cache for a max-stale-ttl interval (default 12 hours).

This allows DNS operators to turn the serve-stale answers in an event of
large authoritative DNS outage.  The caching of the stale answers needs
to be enabled before the outage happens or the feature would be
otherwise useless.

The negative consequence of the default setting is the inevitable
cache-bloat that happens for every and each DNS operator running named.

In this MR, a new configuration option `stale-cache-enable` is
introduced that allows the operators to selectively enable or disable
the serve-stale feature of BIND 9 based on their decision.

The newly introduced option has been disabled by default,
e.g. serve-stale is disabled in the default configuration and has to be
enabled if required.