scorch zap segment merging reuses prealloc'ed PostingsIterator

During zap segment merging, a new zap PostingsIterator was allocated
for every field X segment X term.

This change optimizes by reusing a single PostingsIterator instance
per persistMergedRest() invocation.

And, also unused fields are removed from the PostingsIterator.

index/scorch/segment/zap/dict.go

@@ -44,7 +44,6 @@ func (d *Dictionary) postingsList(term []byte, except *roaring.Bitmap, rv *Posti
 		*rv = PostingsList{} // clear the struct
 	}
 	rv.sb = d.sb
-	rv.term = term
 	rv.except = except
 
 	if d.fst != nil {

index/scorch/segment/zap/merge.go

@@ -164,6 +164,7 @@ func persistMergedRest(segments []*SegmentBase, drops []*roaring.Bitmap,
 	var bufLoc []uint64
 
 	var postings *PostingsList
+	var postItr *PostingsIterator
 
 	rv := make([]uint64, len(fieldsInv))
 	fieldDvLocs := make([]uint64, len(fieldsInv))
@@ -247,7 +248,7 @@ func persistMergedRest(segments []*SegmentBase, drops []*roaring.Bitmap,
 					return nil, 0, err2
 				}
 
-				postItr := postings.Iterator()
+				postItr = postings.iterator(postItr)
 				next, err2 := postItr.Next()
 				for next != nil && err2 == nil {
 					hitNewDocNum := newDocNums[dictI][next.Number()]

index/scorch/segment/zap/posting.go

@@ -28,21 +28,27 @@ import (
 // PostingsList is an in-memory represenation of a postings list
 type PostingsList struct {
 	sb             *SegmentBase
-	term           []byte
 	postingsOffset uint64
 	freqOffset     uint64
 	locOffset      uint64
 	locBitmap      *roaring.Bitmap
 	postings       *roaring.Bitmap
 	except         *roaring.Bitmap
-	postingKey     []byte
 }
 
 // Iterator returns an iterator for this postings list
 func (p *PostingsList) Iterator() segment.PostingsIterator {
-	rv := &PostingsIterator{
-		postings: p,
+	return p.iterator(nil)
+}
+
+func (p *PostingsList) iterator(rv *PostingsIterator) *PostingsIterator {
+	if rv == nil {
+		rv = &PostingsIterator{}
+	} else {
+		*rv = PostingsIterator{} // clear the struct
 	}
+	rv.postings = p
+
 	if p.postings != nil {
 		// prepare the freq chunk details
 		var n uint64