Operators toList and toSortedList now support backpressure

[akarnokd]

Added support for backpressure in toList and toSortedList: they emit their buffered content only when downstream actually requested it.

Few other notes:

• Added overload to toSortedList taking a initialCapacity argument which should help reduce the number of times the buffer needs to be resized.
• The Func2 parameter of toSortedList is now wrapped once per operator instead of once per subscriber.
• Using non-final list buffers which are set to null on completion to not hold onto the buffer.
• toSortedListnow returns a modifiable ArrayList: since it is a handoff procedure, there is no value in forcing a non-modification constraint on the emitted list (similar to toList even before this PR).

[davidmoten]

Cool. I'll be using that initialCapacity and SingleDelayedProducer will be useful too.

[benjchristensen]

Why can't o just be passed into the Subscriber constructor?

[akarnokd]

Because if the result Subscriber receives its Producer via setProducer, the default behavior is to call setProducer on op set by the constructor and thus it can overwrite our SingleDelayedProducer.

[benjchristensen]

Good changes.