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/log.go

@@ -7,6 +7,7 @@ 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"
 )
 
 // log is a logger that is initialized with no output filters.  This
@@ -34,6 +35,7 @@ func UseLogger(logger btclog.Logger) {
 	migration2.UseLogger(logger)
 	migration3.UseLogger(logger)
 	migration4.UseLogger(logger)
+	migration5.UseLogger(logger)
 }
 
 // logClosure is used to provide a closure over expensive logging operations so