With fresh eyes I stopped being confused by P2P protocol free monad stuff,
and got annex.verify=false supported when it's safe to skip verification.

And, I found some cases where resuming a download with annex.verify=false
could let corrupt data into the repository. This is not a new problem; as
well as with the P2P protocol, it could happen when downloading from the
web, and possibly with some external special remotes that support resuming.
So, it seemed best to override annex.verify configuration when
resuming a download.

Also fixed up some progress bar stuff related to the P2P protocol. 
Including dealing with the case where the size of a key being downloaded is
not known until the peer starts sending its data. The progress bar will
now be updated with the size from the P2P protocol, so it can display
a percentage even in this case.

I hope that's the end of the P2P protocol stuff for now.

Today's work was supported by the NSF-funded DataLad project.
