loop: add migration for loopout swaps to fix negative stored costs

Previously we may have stored negative costs for some loop out swaps
which this commit attempts to correct by fetching all completed swap,
calculating the correct costs and overriding them in the database.

loopdb/store_mock.go

@@ -24,6 +24,8 @@ type StoreMock struct {
 	loopInStoreChan  chan LoopInContract
 	loopInUpdateChan chan SwapStateData
 
+	migrations map[string]struct{}
+
 	t *testing.T
 }
 
@@ -39,6 +41,7 @@ func NewStoreMock(t *testing.T) *StoreMock {
 		loopInUpdateChan: make(chan SwapStateData, 1),
 		LoopInSwaps:      make(map[lntypes.Hash]*LoopInContract),
 		LoopInUpdates:    make(map[lntypes.Hash][]SwapStateData),
+		migrations:       make(map[string]struct{}),
 		t:                t,
 	}
 }
@@ -364,12 +367,20 @@ func (s *StoreMock) BatchUpdateLoopOutSwapCosts(ctx context.Context,
 func (s *StoreMock) HasMigration(ctx context.Context, migrationID string) (
 	bool, error) {
 
-	return false, errUnimplemented
+	_, ok := s.migrations[migrationID]
+
+	return ok, nil
 }
 
 // SetMigration marks the migration with the given ID as done.
 func (s *StoreMock) SetMigration(ctx context.Context,
 	migrationID string) error {
 
-	return errUnimplemented
+	if _, ok := s.migrations[migrationID]; ok {
+		return errors.New("migration already done")
+	}
+
+	s.migrations[migrationID] = struct{}{}
+
+	return nil
 }