watchtower: add channelID index

In this commit, a new channel-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 channel
ID (32 bytes). This mapping will help us save space in future when
persisting references to channels.

watchtower/wtdb/version.go

@@ -5,6 +5,7 @@ import (
 	"github.com/lightningnetwork/lnd/kvdb"
 	"github.com/lightningnetwork/lnd/watchtower/wtdb/migration1"
 	"github.com/lightningnetwork/lnd/watchtower/wtdb/migration2"
+	"github.com/lightningnetwork/lnd/watchtower/wtdb/migration3"
 )
 
 // migration is a function which takes a prior outdated version of the database
@@ -33,6 +34,9 @@ var clientDBVersions = []version{
 	{
 		migration: migration2.MigrateClientChannelDetails,
 	},
+	{
+		migration: migration3.MigrateChannelIDIndex,
+	},
 }
 
 // getLatestDBVersion returns the last known database version.