cache statsd rollup types so they can be rolled up
document that bundle.timestamp is millis since epoch.

See corresponding change at https://github.com/gdusbabek/blueflood-statsd-backend/commit/6fc093f814d7405825b63527a150f16931995f81
Don't let one rollup exception kill a batch.
fix serialization bugs with gauges and counters
Merge pull request #224 from gdusbabek/preag_rollup_bugs_1

Fix pre-aggregate bugs discovered while implementing query