feat($rootScope): implement $applyAsync to support combining calls to $apply into a single digest.

It is now possible to queue up multiple expressions to be evaluated in a single digest using
$applyAsync. The asynchronous expressions will be evaluated either 1) the next time $apply or
$rootScope.$digest is called, or 2) after after the queue flushing scheduled for the next turn
occurs (roughly ~10ms depending on browser and application).
feat($http): implement mechanism for coalescing calls to $apply in $http

When multiple responses are received within a short window from each other, it can be wasteful to
perform full dirty-checking cycles for each individual response. In order to prevent this, it is
now possible to coalesce calls to $apply for responses which occur close together.

This behaviour is opt-in, and the default is disabled, in order to avoid breaking tests or
applications.

In order to activate coalesced apply in tests or in an application, simply perform the following
steps during configuration.

   angular.module('myFancyApp', []).
     config(function($httpProvider) {
       $httpProvider.useApplyAsync(true);
     });

OR:

   angular.mock.module(function($httpProvider) {
     $httpProvider.useApplyAsync(true);
   });

Closes #8736
Closes #7634
Closes #5297