2.x: fix Single.using, add unit tests and coverage

[akarnokd]

I've forgotten to create a branch so the first part of todays coverage work landed in 2.x direct

• removed unused methods from various classes
• compacted SpscArrayQueue
• added null checks to SpscLinkedArrayQueue.offer() and MpscLinkedQueue.offer()
• fixed ResourceObserver not calling onStart
• Added direct NotificationLite-emission methods to AppendOnlyLinkedArrayList, updated SerializedObserver and SerializedSubscriber to use it directly instead of the former indirection
• TestObserver.isDisposed now reports true if terminal events were received (2.x Should observer be disposed after the source observable has finished emitting items? #4514)
• ResourceSubscriber to use SubscriptionHelper's deferred Subscription/requesting management
• added unit tests to cover DisposableXObservers
• SerializedObserver and SerializedSubscriber now have proper unit tests each

This current PR fixes Single.using not properly managing the resource and adds unit tests to verify the behavior along with a few extra coverage improvements.

[JakeWharton]

👍

[codecov-io]

Current coverage is 78.64% (diff: 96.92%)

Merging #4516 into 2.x will increase coverage by 0.36%

@@                2.x      #4516   diff @@
==========================================
  Files           507        507          
  Lines         34371      34386    +15   
  Methods           0          0          
  Messages          0          0          
  Branches       5391       5395     +4   
==========================================
+ Hits          26904      27042   +138   
+ Misses         5480       5374   -106   
+ Partials       1987       1970    -17   

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