channeldb/migration13: migrate to mpp structure

This commit migrates the payments in the database to a new structure
that allows for multiple htlcs per payments. The migration introduces a
new sub-bucket that contains a list of htlcs and moves the old single
htlc into that.

channeldb/migration13/log.go

@@ -0,0 +1,14 @@
+package migration13
+
+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
+}