Swap mirror during download when too slow

[nroi]

Even if we try to take care to choose fast mirrors, it's always possible that the chosen mirror ends up being too slow. This is particularly frustrating when a large file is downloaded and the speed suddenly nosedives.

Ideally, cpcache should be able to switch to a (hopefully) better mirror while the download is still in progress. All of this happens transparently to the client. i.e., the client will just see that the speed is first slow, and then suddenly increases.

Things to consider:

• How do we measure "slowness"? Is it the server that's slow, or is it just the user's internet connection? Perhaps we need to record download speeds from previous downloads to make informed decisions.
• If a download happens to be much slower than usual, this does not necessarily mean that the mirror is too slow: It could also be the clients internet connection which is currently under heavy usage.