As part of a performance profiling of mailserver we noticed that most of
the resources on a query are spend decoding the whisper envelope.
This PR changes the way we store envelopes encoding the Topic into the
database key, so we can check that and we are able to publish the
envelope rawValue if it matches.
The change is backward compatible as only newly added envelopes will
have the new key, while old ones will have to be unmarshaled.
Pedro Pombeiro
Andrea Maria Piana