Stop doubling the min-refresh rate if the server doesn't specify one

Now that we only run when live cached elements are on the page, there's no real benefit to this
Add some helper functions
Use Date object instead of hacking date format (which could change)
Add the refresh rate to the server time to make endpoints even more timely
Ignore initial server requests if cached and stale

Staleness is determined by seeing if the Last-Modified value is older than the current request by the length of the refresh rate plus a 10 second grace period.

This check will only run for the initial request to /live_cache_check/1/, as it's assumed that the other urls, which are time based, won't need such aggressive cache busting.

If the cache is stale, this will set the timeStamp variable to the current time and immediately rerun check().