Heartbeat agg rollup producing invalid aggregates.

[WireBaron]

A rollup of hourly data into a daily object gave us the following heartbeat agg object:

(version:1,start_time:722682000000000,end_time:722685600000000,last_seen:722684189000000,interval_len:60000000,num_intervals:3,interval_starts:[722683956000000,722688060000000,722705763000000],interval_ends:[722684249000000,722688237000000,722685600000000])

There are several problems with this aggregate object.

1. This aggregate only covers one hour, where we expect to see a 24 hour aggregate coming from the rollup.
2. The last_seen value is the last value from the first interval.
3. There are only three intervals present, presumably there should be more. (need to verify this)
4. The last interval has an end time prior to its start (this is caused by the aggregate finalizer trimming the end value of the last range to end_time).

We need to make sure that we're updating start_time, end_time, and last_seen when combining ranges. Some more investigation is needed to understand if and why we're dropping intervals.