Add callbacks to gossip.

Support for regexp patterns on keys to have gossip invoke callback
when new infos are added.
We never access this map except by iterating over the whole thing (and never rely on it for deduping since each callback gets a unique regexp object), so it could be a slice of pairs instead of a map.

Actually, are we guaranteed to get a unique regexp object? It looks like we will, but the implementation would be within its rights to cache compiled regexps and return a previously-generated one (python's `re` module does this)