Including resuming and cleanup of incomplete downloads.
Still todo: --fast, --relaxed, importfeed, disk reserve checking,
quvi code cleanup.
This commit was sponsored by Anthony DeRobertis on Patreon.
ReadContent can't update the log, since it reads lazily. This part of
the P2P monad will need to be rethought.
Associated files are heavily sanitized when received from a peer;
they could be an exploit vector.
This commit was sponsored by Jochen Bartl on Patreon.
Note that get --from foo --failed will get things that a previous get --from bar
tried and failed to get, etc. I considered making --failed only retry
transfers from the same remote, but it was easier, and seems more useful,
to not have the same remote requirement.
Noisy due to some refactoring into Types/
