Eliminate the detection time window

[westonruter]

This is the second PR to address #1496.

The detection time window fails to adequately account for page caching so it's gotta go.

When the detection time window is enforced, new URL metrics are only collected if the user happened to visit when there was a page cache MISS or if the user happened to visit a couple seconds from within the time that someone else visited the page and the cache HIT returns a page that is only a couple seconds old. This is problematic, for example, on a site which primarily gets mobile traffic. If mobile visitors always trigger their responses to be cached, then when a desktop visitor comes around the detection time window will have already passed and that URL will be starved of URL Metrics for desktop.

Additionally, on a site that uses a stale-while-revalidate strategy for page caching, it's possible that the cached page is always older than the detection time window, resulting in URL Metrics never being gathered.

Also, a plugin like W3 Total Cache has a "Cache Preload" feature in which it will proactively crawl the site to prime the page cache. Since these requests are not coming from an actual user, they will circumvent a user causing a cache MISS and thus getting a page served within the detection time window.

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Co-authored-by: westonruter <westonruter@git.wordpress.org>
Co-authored-by: felixarntz <flixos90@git.wordpress.org>

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