Assume block request produced what was requested

The composite data pipeline now assumes that the blocks received from a
block request are the blocks it requested instead of parsing the
resulting dataset to find out which blocks were produced.  The old
behavior failed to fetch some blocks when the source and the pipeline
disagreed about the mapping of block indices to blocks.

Change-Id: Ice1a6925dbcd3342ab0a2668daefb8794df79945
Merge topic 'streaming-multiblock-genericio-reader' into master

e23a5b25 Assume block request produced what was requested