Heartbeat aggregate rollup should interpolate aggregates

[WireBaron]

Currently rollup just combines the intervals from each of its component aggregates, meaning a 5 minute heartbeat received three minutes before the end of an interval is only valid for three minutes. If the first heartbeat of the next interval arrives 30 seconds in, the aggregate will mark the 30 seconds prior as dead. We've solved this problem in the interpolate code, but we should also be correcting this automatically in rollup.

As part of fixing this, consider whether any of our other aggregates which support both interpolation and rollup also requires a similar fix.