Changed rules:
* 1.9 Always onSubscribe prior to any other signals
* 1.9 NullPointerException if subscriber is null
* 3.17 Long overflow, effectively unbounded instead of onError
Fixed some more things:
* fixed some FIXME
* Long drop and take
* memory leaks in tck tests, use BeforeClass to create ActorSystem
use AfterClass to shutdown ActorSystem
* more tck tests
* don't emit OnComplete when substream is cancelled
* work around for memory leak in PrefixAndTail
