0.6.4 — a release is visible as soon as WordPress asks

Changed

• The 12-hour cache on a successful release lookup is gone. It was a second cache stacked on top of WordPress's own: core already stores the answer in the update_plugins transient and only calls this code when it decides to refresh — at most hourly on the plugins screen, once a minute on the updates screen. Ours bought nothing on top of that and cost the only thing that matters here. Measured: 0.6.3 was published 14 minutes after a check had run, and the site kept answering no_update: 0.6.2 from a transient holding a stale release for another 12 hours, on an install that had already asked. A release now shows up on the next check WordPress makes.
• A failed lookup is still cached, for 15 minutes. Failure is the case that is also slow — unreachable repo, spent rate limit, DNS — and without the marker every refresh would block a page load on the same timeout. force-check ignores the marker, which is the button someone reaches for in exactly that situation.