watchtower: build channel to sessionIDs index

In this commit, a migration is added that adds an index from channel to
sessionIDs (using the DB-assigned session IDs). This will make it easier
in future to know which sessions have updates for which channels.

watchtower/wtdb/version.go

@@ -11,6 +11,7 @@ import (
 	"github.com/lightningnetwork/lnd/watchtower/wtdb/migration4"
 	"github.com/lightningnetwork/lnd/watchtower/wtdb/migration5"
 	"github.com/lightningnetwork/lnd/watchtower/wtdb/migration6"
+	"github.com/lightningnetwork/lnd/watchtower/wtdb/migration7"
 )
 
 // txMigration is a function which takes a prior outdated version of the
@@ -63,6 +64,9 @@ var clientDBVersions = []version{
 	{
 		txMigration: migration6.MigrateSessionIDIndex,
 	},
+	{
+		txMigration: migration7.MigrateChannelToSessionIndex,
+	},
 }
 
 // getLatestDBVersion returns the last known database version.