lnd+paymentsdb: introduce harness for the payment sql backend

We prepare the code for the sql payment backend. However no
payment db interface method for the sql backend is implemented
yet. This will be done in the following commits. They currently
use the embedded KVStore to satify the build environment.

config_builder.go

@@ -1228,6 +1228,26 @@ func (d *DefaultDatabaseBuilder) BuildDatabase(
 
 			return nil, nil, err
 		}
+
+		// Create the payments DB.
+		//
+		// NOTE:  In the regular build, this will construct a kvdb
+		// backed payments backend. With the test_native_sql tag, it
+		// will build a SQL payments backend.
+		sqlPaymentsDB, err := d.getPaymentsStore(
+			baseDB, dbs.ChanStateDB.Backend,
+			paymentsdb.WithKeepFailedPaymentAttempts(
+				cfg.KeepFailedPaymentAttempts,
+			),
+		)
+		if err != nil {
+			err = fmt.Errorf("unable to get payments store: %w",
+				err)
+
+			return nil, nil, err
+		}
+
+		dbs.PaymentsDB = sqlPaymentsDB
 	} else {
 		// Check if the invoice bucket tombstone is set. If it is, we
 		// need to return and ask the user switch back to using the
@@ -1256,6 +1276,24 @@ func (d *DefaultDatabaseBuilder) BuildDatabase(
 		if err != nil {
 			return nil, nil, err
 		}
+
+		// Create the payments DB.
+		kvPaymentsDB, err := paymentsdb.NewKVStore(
+			dbs.ChanStateDB,
+			paymentsdb.WithKeepFailedPaymentAttempts(
+				cfg.KeepFailedPaymentAttempts,
+			),
+		)
+		if err != nil {
+			cleanUp()
+
+			err = fmt.Errorf("unable to open payments DB: %w", err)
+			d.logger.Error(err)
+
+			return nil, nil, err
+		}
+
+		dbs.PaymentsDB = kvPaymentsDB
 	}
 
 	dbs.GraphDB, err = graphdb.NewChannelGraph(graphStore, chanGraphOpts...)
@@ -1268,29 +1306,6 @@ func (d *DefaultDatabaseBuilder) BuildDatabase(
 		return nil, nil, err
 	}
 
-	// Mount the payments DB which is only KV for now.
-	//
-	// TODO(ziggie): Add support for SQL payments DB.
-	// Mount the payments DB for the KV store.
-	paymentsDBOptions := []paymentsdb.OptionModifier{
-		paymentsdb.WithKeepFailedPaymentAttempts(
-			cfg.KeepFailedPaymentAttempts,
-		),
-	}
-	kvPaymentsDB, err := paymentsdb.NewKVStore(
-		dbs.ChanStateDB,
-		paymentsDBOptions...,
-	)
-	if err != nil {
-		cleanUp()
-
-		err = fmt.Errorf("unable to open payments DB: %w", err)
-		d.logger.Error(err)
-
-		return nil, nil, err
-	}
-	dbs.PaymentsDB = kvPaymentsDB
-
 	// Wrap the watchtower client DB and make sure we clean up.
 	if cfg.WtClient.Active {
 		dbs.TowerClientDB, err = wtdb.OpenClientDB(