Limit the returned db.Querier to the requested time range (#351)

Limit the returned `db.Querier` to the requested time range. Preallocate the `baseChunkSeries.lset` and `baseChunkSeries.chks` slices to the previous series' slice sizes to avoid unnecessary grow slice reallocations.

db.go

@@ -793,7 +793,11 @@ func (db *DB) Querier(mint, maxt int64) (Querier, error) {
 		}
 	}
 	if maxt >= db.head.MinTime() {
-		blocks = append(blocks, db.head)
+		blocks = append(blocks, &rangeHead{
+			head: db.head,
+			mint: mint,
+			maxt: maxt,
+		})
 	}
 
 	sq := &querier{

querier.go

@@ -499,8 +499,8 @@ func (s *baseChunkSeries) Err() error { return s.err }
 
 func (s *baseChunkSeries) Next() bool {
 	var (
-		lset     labels.Labels
-		chkMetas []chunks.Meta
+		lset     = make(labels.Labels, len(s.lset))
+		chkMetas = make([]chunks.Meta, len(s.chks))
 		err      error
 	)