watchtower: add sessionID index

In this commit, a new session-ID index is added to the tower client db
with the help of a migration. This index holds a mapping from a
db-assigned-ID (a uint64 encoded using BigSize encoding) to real session
ID (33 bytes). This mapping will help us save space in future when
persisting references to sessions.

watchtower/wtdb/version.go

@@ -10,6 +10,7 @@ import (
 	"github.com/lightningnetwork/lnd/watchtower/wtdb/migration3"
 	"github.com/lightningnetwork/lnd/watchtower/wtdb/migration4"
 	"github.com/lightningnetwork/lnd/watchtower/wtdb/migration5"
+	"github.com/lightningnetwork/lnd/watchtower/wtdb/migration6"
 )
 
 // txMigration is a function which takes a prior outdated version of the
@@ -59,6 +60,9 @@ var clientDBVersions = []version{
 	{
 		txMigration: migration5.MigrateCompleteTowerToSessionIndex,
 	},
+	{
+		txMigration: migration6.MigrateSessionIDIndex,
+	},
 }
 
 // getLatestDBVersion returns the last known database version.