Include patch from graphite-project/carbon#109 by @pgul

Aggregated metrics sometimes are not correct

In some rare cases aggregated metrics are much less then sum() of its
components.

This caused when carbon-aggregator sends the same metric/timestamp to
carbon-cache more then once with different values (partially collected and then
final value), these values queued by carbon-cache, fetched from queue by one
pop() call, and stored by one whisper.update_many() call. Whisper sort these
datapoints by timestamps, and it's possible that within the same timestamp
earlier (partial) aggregated metric will be stored in wsp file instead of final
value.

Using branch 0.9.x.