In the past, we only have changes-since filter, we called it as
timestamp-filter, but now we will add a changes-before filter,
the timestamp-filter is not clear anymore.
In this patch, we rename timestamp to changes-since.
Change-Id: Ieee912e197d17a5ffca7460f3b977316cd21813e
blueprint: support-to-query-nova-resources-filter-by-changes-before
Yikun Jiang