Fixed the issues of takeLast(items, 0) and null values

[zsxwing]

Hi,

There are two issues about takeLast #85 #140. The essential cause is LinkedBlockingDeque.

1. count == 0
   takeLast in RxJava will throw an exception when count == 0, as LinkedBlockingDeque rejects count <= 0. However, in c#, TakeLast with 0 count is valid. The following codes is OK in C# (no exception and do nothing):

            var source = Observable.Return(1).TakeLast(0);
            source.Subscribe(
                x => 
                    Console.WriteLine("subscriber got " + x)
            );
            Console.ReadLine();

1. null values
   LinkedBlockingDeque requires the elements can not be null but an observable can emit a null value.

I used ReentrantLock and LinkedList to replace LinkedBlockingDeque. Please take a look. Thanks.

[cloudbees-pull-request-builder]

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[benjchristensen]

Merging as this seems correct ... but I question whether we need any of this synchronization in here since this operator would never be multi-threaded. It can count on the Rx contract to have sequential onNext events.

Thus I wonder if the performance hit of all this synchronization is a concern or necessary. I'm favoring functionality and merging now ... but something to consider for the future.