watchtower/wtdb: add tower-to-session index entry for all towers

In this commit, a small migration is added to the watchtower client DB
to ensure that there is an entry in the towerID-to-sessionID index for
all towers in the db regardless of if they have sessions or not. This is
required as a follow up to migration 1 since that migration only created
entries in the index for towers that had associated sessions which would
lead to "tower not found" errors on start up.

watchtower/wtdb/version.go

@@ -9,6 +9,7 @@ import (
 	"github.com/lightningnetwork/lnd/watchtower/wtdb/migration2"
 	"github.com/lightningnetwork/lnd/watchtower/wtdb/migration3"
 	"github.com/lightningnetwork/lnd/watchtower/wtdb/migration4"
+	"github.com/lightningnetwork/lnd/watchtower/wtdb/migration5"
 )
 
 // txMigration is a function which takes a prior outdated version of the
@@ -55,6 +56,9 @@ var clientDBVersions = []version{
 			migration4.DefaultSessionsPerTx,
 		),
 	},
+	{
+		txMigration: migration5.MigrateCompleteTowerToSessionIndex,
+	},
 }
 
 // getLatestDBVersion returns the last known database version.