2 new tests for OnSubscribeToObservableFuture

1) Creating an Observable from an already-cancelled Future should emit nothing (equivalent to Observable.never())
   (it currently emits onError(CancellationException)
2) Creating an Observable from a Future then unsubscribeing immediately before Future.get()
   unblocks should be an Observable.never().  This test already passes.
Modify OnSubscribeToObservableFuture to return early if unsubscribe without emitting anything
add annotations about default schedulers to javadocs for Observable operators
Fix and Update JMH Perf Tests

- The tests were wrong and re-using a single Subscriber instance which meant they weren't really testing much.
- Same with the CountDownLatch which meant they weren't waiting if async.
- Added several SerializePerf and PerfTransform tests
Merge pull request #1293 from benjchristensen/perf-tests

Fix and Update JMH Perf Tests
Merge pull request #1291 from mattrjacobs/check-unsubscribe-within-observable-from-future

Check unsubscribe within observable from future