cache_failures: allow client code to instruct couch_mgr to cache
failures, to avoid doing lookups continually for docs that won't exist
in the near-term.
cache_failures: cache number lookup failures

when an offnet route_req comes in for an unknown number, many whapps
will attempt to resolve the account-id of the dialed number. Since the
number doesn't exist, this will cause lots of 404 requests from
Couch (and wasted time waiting for the 404). By caching the failure, we
can avoid the lookup after the first failure. The failure will be
flushed once the doc id created/modified/deleted.
cache_failures: allow caching only on certain failures
cache_failures: handle undefined or other values by not caching
Merge pull request #519 from 2600hz/cache_failures

Cache failures