Push scalability (#3080)

* Update status through increment
* adds support for incrementing nested keys
* fix issue when having spaces in keys for ordering
* Refactors PushController to use worker
* Adds tests for custom push queue config
* Makes PushController adapter independant
* Better logging of _PushStatus in VERBOSE

spec/PushWorker.spec.js

@@ -0,0 +1,57 @@
+var PushWorker = require('../src').PushWorker;
+var Config = require('../src/Config');
+
+describe('PushWorker', () => {
+  it('should run with small batch', (done) => {
+    const batchSize = 3;
+    var sendCount = 0;
+    reconfigureServer({
+      push: {
+        queueOptions: {
+          disablePushWorker: true,
+          batchSize
+        }
+      }
+    }).then(() => {
+      expect(new Config('test').pushWorker).toBeUndefined();
+      new PushWorker({
+        send: (body, installations) => {
+          expect(installations.length <= batchSize).toBe(true);
+          sendCount += installations.length;
+          return Promise.resolve();
+        },
+        getValidPushTypes: function() {
+          return ['ios', 'android']
+        }
+      });
+      var installations = [];
+      while(installations.length != 10) {
+        var installation = new Parse.Object("_Installation");
+        installation.set("installationId", "installation_" + installations.length);
+        installation.set("deviceToken","device_token_" + installations.length)
+        installation.set("badge", 1);
+        installation.set("deviceType", "ios");
+        installations.push(installation);
+      }
+      return Parse.Object.saveAll(installations);
+    }).then(() => {
+      return Parse.Push.send({
+        where: {
+          deviceType: 'ios'
+        },
+        data: {
+          alert: 'Hello world!'
+        }
+      }, {useMasterKey: true})
+    }).then(() => {
+      return new Promise((resolve) => {
+        setTimeout(resolve, 500);
+      });
+    }).then(() => {
+      expect(sendCount).toBe(10);
+      done();
+    }).catch(err => {
+      jfail(err);
+    })
+  });
+});