The satellite now returns the last interval_end_time for each
daily storage usage.
We need to store the interval_end_time in the storage usage cache.
Also, renamed interval_start to timestamp to avoid ambiguity since
the interval_start only stores just the date/day returned by the
satellite.
Updates https://github.com/storj/storj/issues/4178
Change-Id: I94138ba8a506eeedd6703787ee03ab3e072efa32
