invoices: jit insert key send invoices

This commit adds handling code for the key send custom record. If this
record is present and its hash matches the payment hash, invoice
registry will insert a new invoice into the database "just in time". The
subsequent settle flow is unchanged. The newly inserted invoice is
picked up and settled. Notifications will be broadcast as usual.

invoices/test_utils_test.go

@@ -21,7 +21,8 @@ import (
 )
 
 type mockPayload struct {
-	mpp *record.MPP
+	mpp           *record.MPP
+	customRecords record.CustomSet
 }
 
 func (p *mockPayload) MultiPath() *record.MPP {
@@ -29,7 +30,13 @@ func (p *mockPayload) MultiPath() *record.MPP {
 }
 
 func (p *mockPayload) CustomRecords() record.CustomSet {
-	return make(record.CustomSet)
+	// This function should always return a map instance, but for mock
+	// configuration we do accept nil.
+	if p.customRecords == nil {
+		return make(record.CustomSet)
+	}
+
+	return p.customRecords
 }
 
 var (