optimise tag code

Removes contact tags, caches known tags for a tag type (in a fashion that should pick up old data OK as well) and removes the overlaying of Redis operations onto a Set object that didn't really fit the rest of the code. Not perfect, but faster than before due to minimal use of the Redis `KEYS` command.

Addresses https://github.com/flapjack/flapjack/issues/652.
is this line doing anything?