Avoid holding data elements alive via stack frame gc roots.

[scwhittle]

This is accomplished by changing to iterators which throw away elements that have been passed as well as just referring to the inputstream of the element instead of the entire stream. If the ByteString is based upon a ByteBuffer this will allow blocks that have been advanced past to be gc'd.

This isn't done for the inlined elements to process data as that request is kept alive in many other places. I investigated fixing it but it resulted in some complicated code and since inlining is only performed for small-enough inputs it doesn't seem worth it.

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[scwhittle]

R: @robertwb

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[scwhittle]

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