mutable-retrieve should accept good shares from servers with bad shares

[warner]

retrieve.py is currently rather punitive when it detects corrupt shares: it stops using *any* shares from the same server. This doesn't have much effect when there are plenty of servers and the shares are spread thinly, but in small personal grids, a single corrupt share could disqualify enough shares that the retrieve fails.

I think this behavior was copied from the old immutable downloader. The new immutable downloader doesn't do this: it treats each share independently.

To fix this, _mark_bad_share and _remove_reader should remove just the one share from self.remaining_sharemap, instead of all shares with the same server. We also need to look for other calls to _remove_reader(), since I think there may be some which *are* supposed to stop all shares from the same server.