scorch zap merge uses array for docTermMap with no sorting
scorch zap merge uses array for docTermMap with no sorting

Instead of sorting docNum keys from a hashmap, this change instead
iterates from docNum 0 to N and uses an array instead of hashmap.
The array is also reused across outer loop iterations.

This optimizes for when there's a lot of structural similarity between
docs, where many/most docs have the same fields.  i.e., beers,
breweries.  If every doc has completely different fields, then this
change might produce worse behavior compared to the previous sparse
hashmap approach.

blevesearch/bleve,steveyen/bleve,steveyen/bleve,blevesearch/bleve