Introduce gossipV1 and gossipV2 package-level aliases in sql_store.go
to reduce verbosity in version switch statements. Leave hard-coded
v1/v2 call sites untouched so remaining upgrades are obvious.
In other words, it is now easy to see where our remaining work in the
sql_store.go file is by just searching for instances of
`lnwire.GossipVersion1`.