Improve eachBatch method (#1179)

* Change the promises order of the eachBatch method to allow querying while executing the callback on the previous result as the same time.

* Change the "finished" test to take into account the find mock of the tests

src/ParseQuery.js

@@ -923,18 +923,23 @@ class ParseQuery {
     }
 
     let finished = false;
+    let previousResults = [];
     return continueWhile(() => {
       return !finished;
-    }, () => {
-      return query.find(findOptions).then((results) => {
-        return Promise.resolve(callback(results)).then(() => {
-          if (results.length >= query._limit) {
-            query.greaterThan('objectId', results[results.length - 1].id);
-          } else {
-            finished = true;
-          }
-        });
-      });
+    }, async () => {
+      const [results] = await Promise.all([
+        query.find(findOptions),
+        Promise.resolve(previousResults.length > 0 && callback(previousResults))
+      ]);
+      if (results.length >= query._limit) {
+        query.greaterThan('objectId', results[results.length - 1].id);
+        previousResults = results;
+      } else if (results.length > 0) {
+        await Promise.resolve(callback(results));
+        finished = true;
+      } else {
+        finished = true;
+      }
     });
   }