Disable the shared cache between resolvers for now.

Disable the shared cache between resolvers for now.

Since the latest libunbound update the frontend process would segfault
about once a day on one of my MX servers with what looks like a
use-after-free deep inside of libunbound.

Maybe we are poking too much at internals and the shared cache is too
much of a hack. #ifdef for now to ease investigation, but it is
possible that this code just has to go.