Add first and last sample timestamp

When listing resources, it's useful to know where it started and where it
stopped. We don't have this precise information, but we can at least say
when we get the first and last timestamp.

Blueprint: api-v2-improvement

Change-Id: Icf678820bf7c3e6e12288b15623b653d9a942f21
Signed-off-by: Julien Danjou <julien@danjou.info>