Fix pre-aggregate bugs discovered while implementing query

Some of these were lost during the PR99 refactorings and regroupings.  Others are just bugs.

* rollup type was not getting saved during ingestion. This prevented rollups from being read from the right CF.
* ingestion timestamps were being sent as seconds-since-epoch. I decided to make this fix at the [backend level](https://github.com/gdusbabek/blueflood-statsd-backend/commit/6fc093f814d7405825b63527a150f16931995f81), and document that fact that `Bundle.timestamp` is in millis.
* log an error on RTE during batch rollups instead of killing the whole batch.
* gauges and counters were being serialized and tested improperly.