Performance group cancel.

[kinarobin]

1. Once you want cancel group, the _groupToOperationReferences contains all object,

for (id <PINOperationReference>operationReference in [_groupToOperationReferences objectEnumerator]) 
{
     if ([_operationQueue cancelOperation:operationReference]) {
            dispatch_group_leave(_group);
     }  
}

Execute all object's cancelOperation is so expensive, So I remove single operationReference when single operation had finished.

2. If the group start, it's not necessary to nil out the containers.

  _operations = nil;
  _operationPriorities = nil;
  _operationReferences = nil;

Just remove all objects. Thus support adding operations after completion or cancelation.

[ghost]

	1 Warning
:warning:	Any source code changes should have an entry in CHANGELOG.md or have #trivial in their title.

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