Reuse already transferred segments from other torrents

[krichter]

In a scenario where torrents with a lot of similar files are added to deluge (e.g. different versions of an operating system or different versions with large sets of files in common) it'd be useful to maintain a list of all segment checksums of already transfered data and search segments which ought to be transfered in that list and use them if present instead of transferring them.

In the non-torrent world this is done by zsync which usually can reuse between 25 to 40% of data when requested to transfer a newer version of Ubuntu and use the previous one as input.