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/log.go

@@ -5,6 +5,7 @@ import (
 	"github.com/lightningnetwork/lnd/build"
 	"github.com/lightningnetwork/lnd/watchtower/wtdb/migration1"
 	"github.com/lightningnetwork/lnd/watchtower/wtdb/migration2"
+	"github.com/lightningnetwork/lnd/watchtower/wtdb/migration3"
 )
 
 // log is a logger that is initialized with no output filters.  This
@@ -30,6 +31,7 @@ func UseLogger(logger btclog.Logger) {
 	log = logger
 	migration1.UseLogger(logger)
 	migration2.UseLogger(logger)
+	migration3.UseLogger(logger)
 }
 
 // logClosure is used to provide a closure over expensive logging operations so