rec: Log protobuf messages for cache hits. Add policy tags in gettag()

rec: Log protobuf messages for cache hits. Add policy tags in gettag()

gettag()` can now return an optional policy tags table in addition to
the existing `tag` integer.
Question and response protobuf messages are now sent even on cache hits.
When protobuf logging is enabled, the protobuf response message is
added to the cache and retrieved together with the response.