payments/migration1: wire KV→SQL migration in the main pkg

Hook the payments KV→SQL migration into the SQL migration config.
The migration is still only available when building with the build tag
"test_native_sql".

Moreover a tombstone protection similar to the invoice migration is added
to prevent re-runningi with the KV backend  once migration completes.

sqldb/migrations_dev.go

@@ -8,4 +8,18 @@ var migrationAdditions = []MigrationConfig{
 		Version:       12,
 		SchemaVersion: 10,
 	},
+	{
+		Name:          "000011_payment_duplicates",
+		Version:       13,
+		SchemaVersion: 11,
+	},
+	{
+		Name:          "kv_payments_migration",
+		Version:       14,
+		SchemaVersion: 11,
+		// A migration function may be attached to this
+		// migration to migrate KV payments to the native SQL
+		// schema. This is optional and can be disabled by the
+		// user if necessary.
+	},
 }