migration30: add iterator to assist migration

This commit adds several utility functions to assist the migration. In
particular, an updateLocator is added to gives us the next un-migration
position in our buckets. This locator helps us to continue the job
in case of an interrupted migration. It also serves as an indicator on
whether the migration is finished or not.

channeldb/migration30/log.go

@@ -0,0 +1,14 @@
+package migration30
+
+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
+}