channeldb: add gossiper message store key migration

In this commit, we introduce a migration for the message store
sub-bucket that will migrate all keys within it to a new key format.
This new key format is composed of the peer's public key, followed by
the short channel ID, followed by the message type. This migration is
needed in order to provide backwards-compatibility with messages that
were previously stored before the introduction of the new key format.

channeldb/db.go

@@ -90,6 +90,13 @@ var (
 			number:    7,
 			migration: migrateOptionalChannelCloseSummaryFields,
 		},
+		{
+			// The DB version that changes the gossiper's message
+			// store keys to account for the message's type and
+			// ShortChannelID.
+			number:    8,
+			migration: migrateGossipMessageStoreKeys,
+		},
 	}
 
 	// Big endian is the preferred byte order, due to cursor scans over