upside_down merge backIndexRow concurrently
upside_down merge backIndexRow concurrently

Previously, the code would gather all the backIndexRows before
processing them.  This change instead merges the backIndexRows
concurrently on the theory that we might as well make progress on
compute & processing tasks while waiting for the rest of the back
index rows to be fetched from the KVStore.

steveyen/bleve,blevesearch/bleve,slavikm/bleve,slavikm/bleve,ikawaha/bleve,steveyen/bleve,blevesearch/bleve,ikawaha/bleve