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/migration7/log.go

@@ -0,0 +1,14 @@
+package migration7
+
+import (
+	"github.com/btcsuite/btclog"
+)
+
+// log is a logger that is initialized as disabled.  This means the package will
+// not perform any logging by default until a logger is set.
+var log = btclog.Disabled
+
+// UseLogger uses a specified Logger to output package logging info.
+func UseLogger(logger btclog.Logger) {
+	log = logger
+}