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
- ordering is not preserved by shufflers in GraphOpsIntegrationSpec
- larger tolerance is needed in GraphBalanceSpec since balancing does
not keep track of previous imbalances
- also add Source.repeat(elem)
* FlowMaterializer is now the actor independent interface
* ActorFlowMaterializer is the actor based interface
* MaterializerSettings renamed to ActorFlowMaterializerSettings
* impl.ActorBasedFlowMaterializer renamed to impl.ActorFlowMaterializerImpl
* Optimizations included in ActorFlowMaterializerSettings
* Note that http is using FlowMaterializer in api, but I suspect that it
will currently only run with a ActorFlowMaterializer
* StreamTcp and Http extensions now return Flows and Sources that can be materialized later
* Flow can now be completed with another flow to be turned into a runnable flow
