fix data race in doc id search

the implementation of the doc id search requires that the list
of ids be sorted.  however, when doing a multisearch across
many indexes at once, the list of doc ids in the query is shared.
deeper in the implementation, the search of each shard attempts
to sort this list, resulting in a data race.

this is one example of a potentially larger problem, however
it has been decided to fix this data race, even though larger
issues of data owernship may remain unresolved.

this fix makes a copy of the list of doc ids, just prior to
sorting the list.  subsequently, all use of the list is on the
copy that was made, not the original.

fixes #518

index/upsidedown/reader.go

@@ -190,15 +190,18 @@ func newUpsideDownCouchDocIDReader(indexReader *IndexReader) (*UpsideDownCouchDo
 }
 
 func newUpsideDownCouchDocIDReaderOnly(indexReader *IndexReader, ids []string) (*UpsideDownCouchDocIDReader, error) {
+	// we don't actually own the list of ids, so if before we sort we must copy
+	idsCopy := make([]string, len(ids))
+	copy(idsCopy, ids)
 	// ensure ids are sorted
-	sort.Strings(ids)
+	sort.Strings(idsCopy)
 	startBytes := []byte{0x0}
-	if len(ids) > 0 {
-		startBytes = []byte(ids[0])
+	if len(idsCopy) > 0 {
+		startBytes = []byte(idsCopy[0])
 	}
 	endBytes := []byte{0xff}
-	if len(ids) > 0 {
-		endBytes = incrementBytes([]byte(ids[len(ids)-1]))
+	if len(idsCopy) > 0 {
+		endBytes = incrementBytes([]byte(idsCopy[len(idsCopy)-1]))
 	}
 	bisr := NewBackIndexRow(startBytes, nil, nil)
 	bier := NewBackIndexRow(endBytes, nil, nil)
@@ -207,7 +210,7 @@ func newUpsideDownCouchDocIDReaderOnly(indexReader *IndexReader, ids []string) (
 	return &UpsideDownCouchDocIDReader{
 		indexReader: indexReader,
 		iterator:    it,
-		only:        ids,
+		only:        idsCopy,
 		onlyMode:    true,
 	}, nil
 }