fixing method setIndexesWithSchemaFormat (#4454)

Fixing invalid database code in method `setIndexesWithSchemaFormat`:

* It must be a transaction, not a task, as it executes multiple database changes
* It should contain the initial queries inside the transaction, providing the context, not outside it;
* Replaced the code with the ES6 Generator notation
* Removing the use of batch, as the value of the result promise is irrelevant, only success/failure that matters

Author: vitaly-t

src/Adapters/Storage/Postgres/PostgresStorageAdapter.js

@@ -621,7 +621,7 @@ export class PostgresStorageAdapter implements StorageAdapter {
     });
   }
 
-  setIndexesWithSchemaFormat(className: string, submittedIndexes: any, existingIndexes: any = {}, fields: any, conn: any): Promise<void> {
+  setIndexesWithSchemaFormat(className: string, submittedIndexes: any, existingIndexes: any = {}, fields: any, conn: ?any): Promise<void> {
     conn = conn || this._client;
     const self = this;
     if (submittedIndexes === undefined) {