make ROLLUP_DELAY_MILLIS configurable.

Defaults to 5 mins.

BF is naturally sensitive to out-of-order metrics. We can tolerate them every once in a while. But if late metrics arrive as a constant stream, we end up doing more rollups than we should.

A simple fix for this is to make the wait-threshold for rolling up metrics configurable. Set it to a value where you are confident you will not receive a lot of metrics older than.

Obviously, this affects how soon rollups appear.

We need to solve this in a better way in the long term.