Write the last received value for a datapoint

Keep points in input order. Sort points to have proper time order as order
is arbitrary. See: items()
http://docs.python.org/2/library/stdtypes.html#mapping-types-dict

Also see:
https://bugs.launchpad.net/carbon/+bug/973420
https://github.com/graphite-project/whisper/pull/33

Reverse reordered values instead of keeping values of same timestamp in
input order. With previous dict method you end up with first value.