1.x: benchmark just and Single

[akarnokd]

This PR adds a comparison benchmark for working with Observable.just and Single.just.

Results: (i7 4790, Win 7 x64, Java 8u92)

• one: custom Observable emitting a single item with backpressure
• scalar: Observable.just(1)
• scalarHidden: just(1).asObservable()
• single: Single.just(1)
• singleHidden: wrapping with Single.create
• plain: just subscribe
• Concat: use concatMap to map it to a scalar (operator not available in Single)
• Merge : use flatMap to map it to a scalar/singe
• Switch : use switchMap to map it to a scalar (operator not available in Single)

My guess is that single is slower because that Observable.OnSubscribe vs Single.OnSubscribe wrapping game.