rxjava-scala improvements and reimplemented the amb operator

[zsxwing]

This PR did the following improvements:

• Added amb, delay, delaySubscription in rxjava-scala.
• Reimplemented the amb operator.

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RxJava-pull-requests #887 SUCCESS
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[akarnokd]

I wonder if this could be simplified to only a single get() call:

int ch = choice.get();
if (ch == NONE) {
    return choice.compareAndSet(NONE, index);
}
return ch == index;

Since this method is called for every onNext() for the winner, this change might reduce the overhead by 2; haven't benchmarked it. In addition, switching to a volatile int choice + double-check synchronization might be a bit faster even. Thoughts?

[zsxwing]

Not quite sure about "volatile + double-check". If JVM can inline "choice.get()", it should be same as "accessing a volatile var".

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RxJava-pull-requests #888 SUCCESS
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