watchtower/wtdb: add new towerID-to-sessionID index

This commit adds a new towerID-to-sessionID index to the wtclient DB.
The commit also contains the necessary migration required in order to
build the index for an existing client.
This index will greatly improve the lookup of sessions for a given tower
ID.

watchtower/wtdb/log.go

@@ -3,6 +3,7 @@ package wtdb
 import (
 	"github.com/btcsuite/btclog"
 	"github.com/lightningnetwork/lnd/build"
+	"github.com/lightningnetwork/lnd/watchtower/wtdb/migration1"
 )
 
 // log is a logger that is initialized with no output filters.  This
@@ -26,6 +27,7 @@ func DisableLog() {
 // using btclog.
 func UseLogger(logger btclog.Logger) {
 	log = logger
+	migration1.UseLogger(logger)
 }
 
 // logClosure is used to provide a closure over expensive logging operations so