[iOS] Stats: Caching improvements

[frosty]

When monitoring the app's network traffic using Charles Proxy, we can see that when you switch between tabs or dates in the Stats view we seem to make requests to the server every single time even if we've just fetched data for the current view.

We should improve caching and make better use of the caching we have in place to ensure that we don't continually hit the network. Each of these views has pull to refresh, so users can always manually fetch the latest data.

[frosty]

After walking through the code, it seems that we already have good caching in place but we can make some improvements to smooth out the user experience:

• We currently hit the network every time we revisit the screens. Android has a TTL of 5 minutes for data in the stats screen, so we'll implement the same here. If cached data exists and it's < 5 mins since we last reloaded, we'll show the cached data. If users want newer data in that time, they can still pull to refresh.
• In the Period screens, we ghost views on everything when we reload even if we already had cached data. Instead, we'll not show ghost loading views if we already have cached data to display.

[staskus]

I'm cleaning up issues related to Stats Revamp and Stats.

@frosty , I'm unassigning you from the task and we'll see if these suggestions are still relevant going forward.

[staskus]

When updating Traffic/Insights tabs we're looking and implementing optimization and caching on a case-by-case basis. I'm closing this issue.