In this commit, we check that the extra bytes appended to gossip
messages contain valid TLV streams. We do this here for:
- channel_announcement
- channel_announcement_2
- channel_update
- channel_update_2
- node_announcement
This is in preparation for the SQL version of the graph store which will
normalise TLV streams at persistence time.
In this commit, we introduce the `V1Store` interface which the existing
`graphdb.KVStore` implements today. The idea is to eventually create a
SQL DB backed implementation of this interface.
