Redesign the unreachable primaries cache

Redesign the unreachable primaries cache

The cache for unreachable primaries was added to BIND 9 in 2006 via
1372e172d0e0b08996376b782a9041d1e3542489. It features a 10-slot LRU
array with 600 seconds (10 minutes) fixed delay. During this time, any
primary with a hiccup would be blocked for the whole block duration
(unless overwritten by a different entry).

As this design is not very flexible (i.e. the fixed delay and the fixed
amount of the slots), redesign it based on the badcache.c module, which
was implemented earlier for a similar mechanism.

The differences between the new code and the badcache module were large
enough to create a new module instead of trying to make the badcache
module universal, which could complicate the implementation.

The new design implements an exponential backoff for entries which are
added again soon after expiring, i.e. the next expiration happens in
double the amount of time of the previous expiration, but in no more
time than the defined maximum value.

The initial and the maximum expiration values are hard-coded, but, if
required, it should be trivial to implement configurable knobs.