Reworked the filtering and rebuilding process

- There are no longer any time filters on the rebuild_metrics method. All
  metrics will be loaded into memory.
- As a result the time filter variable on the rebuild_metrics method has been
  removed.
- The date and time filters have been retained on the slowest_actions methods,
  but slightly renamed to help with readability, and reversed to aid in the
  speed of the filtering unless a time filter is explicitly applied.
- Added a comment about the @metrics hash being returned as an instance of
  HashWithIndifferentAccess, though I realize now that I didn't actually enforce
  that just yet.