watchtower: build channel to sessionIDs index

In this commit, a migration is added that adds an index from channel to
sessionIDs (using the DB-assigned session IDs). This will make it easier
in future to know which sessions have updates for which channels.

watchtower/wtdb/log.go

@@ -9,6 +9,7 @@ import (
 	"github.com/lightningnetwork/lnd/watchtower/wtdb/migration4"
 	"github.com/lightningnetwork/lnd/watchtower/wtdb/migration5"
 	"github.com/lightningnetwork/lnd/watchtower/wtdb/migration6"
+	"github.com/lightningnetwork/lnd/watchtower/wtdb/migration7"
 )
 
 // log is a logger that is initialized with no output filters.  This
@@ -38,6 +39,7 @@ func UseLogger(logger btclog.Logger) {
 	migration4.UseLogger(logger)
 	migration5.UseLogger(logger)
 	migration6.UseLogger(logger)
+	migration7.UseLogger(logger)
 }
 
 // logClosure is used to provide a closure over expensive logging operations so