* App and index downloads fall back to a list of mirrors defined
by the repository.
* The changes have been made trying to keep the original download
code untouched, and only using the mirror logic when the download
fails due to a connection error / timeout.
* The mirrors are tried in a randomized manner, and with proper
timeouts. The download is aborted after the tries exceed the
number of mirrors, times 3 for a total of 3 different timeout
values (10s, 30s, and 1m)
* The mirror code isn't used for any images yet, most of which is
handled by an external library.
Closes: #35
a160476a14