1st & last sample timestamps in Resource representation

Fixes bug 1262335

Previously, the first and last sample timestamps associated
with a resource were calculated by the storage driver, but
then discarded.

Now, we include these values in the API resource representation.

Change-Id: I3cf6081e1ac1294083f6326f85dabebab0d02203
(cherry picked from commit 6a9dc97f962c288f5037d59d05a771823eb4bcdd)