BUG: Move minutely performance period end time forward in time.

The end time of the performance period during minutely emission
should move forward with the events' dt, not be static.
BUG: Fix extra minutely performance period during minute performance.

Prevent an extra performance result with the timestamp of the midnight of
the day from being emitted.

Fix by setting the `saved_dt` value with the dt of the first event,
before entering into the main performance loop, otherwise a performance
result with a midnight timestamp and data from just the first event is
emitted.