BFTask retain cycle

There is a way to introduce retain cycle and get a BFTask object leaked. Just create a task that is never finished with result/error or cancelled. Here is the example:

[[self asyncTask] continueWithSuccessBlock:^id _Nullable(BFTask * _Nonnull t) {
    ...
}];

- (BFTask *)syncTask {
    BFTaskCompletionSource *taskSource = [BFTaskCompletionSource taskCompletionSource];
    return taskSource.task;
}

The BFTask object is captured in the executionBlock block of the continueWithExecutor:block:cancellationToken: method.

I'd propose to get rid of self reference capturing and cancel the task on deallocation (if needed).