fix descending queries

* did not handle cached values correctly
* sort shards by time in either ascending or descending
  order depending on the RPC request ordering to ensure they
  are traversed in the correct order.

services/storage/store.go

@@ -3,6 +3,7 @@ package storage
 import (
 	"context"
 	"errors"
+	"sort"
 	"strings"
 	"time"
 
@@ -71,6 +72,12 @@ func (s *Store) Read(ctx context.Context, req *ReadRequest) (*ResultSet, error)
 		return nil, nil
 	}
 
+	if req.Descending {
+		sort.Sort(sort.Reverse(meta.ShardGroupInfos(groups)))
+	} else {
+		sort.Sort(meta.ShardGroupInfos(groups))
+	}
+
 	shardIDs := make([]uint64, 0, len(groups[0].Shards)*len(groups))
 	for _, g := range groups {
 		for _, si := range g.Shards {