Keep worker thread running after an exception

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Keep track of all replicas of an object, and retry

Choose replicas randomly. If one replica fails due to some kind of
IOError (e.g. the file is no longer there), try another replica.
Retry up to 10 times with a 0.1 second delay if another error (like a
temporary ssh failure) occurs.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>