Keep single-use routes for a few seconds before pruning them

[falkoschindler]

As discussed in issue #4016, ui.download can fail on the auto-index page if multiple tabs try to download from a single-use route at the same time. The first download will remove the route and subsequent downloads will fail.

This PR addresses this problem by keeping used single-use routes in a dictionary with an expiration timestamp. A pruning loop will check this dictionary every 10 seconds and remove outdated routes.

Note that simply delaying the removal after creating a single-use route did not work. The path of a single-use route is basically a hash of the filepath and, thus, not unique to a single call to ui.download. Therefore a route could be used again and again, so we need to keep it as long as ui.download is still being called.