Lock-free, MPSC-queue based, fast-path serializing Observer.
Merge branch 'SerializedObserverPerf521' of github.com:akarnokd/RxJava into util-with-mpsc
Revert SerializedObserver Changes

Not ready to merge these.
Move MPSC Queue to rx.internal.util
Merge pull request #1284 from benjchristensen/util-with-mpsc

Manual merge of Lock-free, MPSC-queue based