This commit adds boolean flag params to the scorch
PostingsList.Iterator() method, so that the caller can specify whether
freq/norm/locs information is needed or not.
Future changes can leverage these params for optimizations.
This is a stepping stone to allow easier future comparisons of field
maps and potential merge optimizations.
In bleve-blast tests on a 2015 macbook (50K wikipedia docs, 8
indexers, batch size 100, ssd), this does not seem to have a distinct
effect on indexing throughput.
-VisitableDocValueFields API for persisted DV field list
-making dv configs overridable at field level
-enabling on the fly/runtime un inverting of doc values
-few UT updates
