Change download expiry behaviour

[noahc3]

The current behaviour for expiring downloads (ie. deciding when to delete the files from the server and kill the download) causes problems with download managers. This happens because, currently, the file is deleted immediately after the first download stream is closed.

• This causes problems for people who manually add files to download managers as they have to cancel the download then transfer the link to the download manager, but the file is already deleted when they cancel the download.
• This causes problems with multiple simultaneous download streams as the moment one of the streams closes the file is deleted and all other streams reach unexpected EoF.

The behaviour should be reworked to reflect the following:

• If no download stream begins within 180 seconds of the file generation being completed, delete the file.
• When a download stream closes, if there are no other active streams, wait 180 seconds before deleting the file.
• If within the 180 second period for the above point a new download stream starts, pause and reset the timer and repeat the behavior in the above point.

Target: SDSetup Backend Rewrite (Late July)

[noahc3]

851c753c8d63d3e7f3d2494fac2a4491c1a564a0