make git-remote-daemon ssh transport robust

* Remote system might be available, and connection get lost. Should
  reconnect, but needs to avoid bad behavior (ie, constant reconnect
  attempts.) Use exponential backoff.

* Detect if old system had a too old git-annex-shell, and show the user
  a nice message in the webapp. Required parsing error messages, so perhaps
  this code shoudl be removed once enough time has passed..

* Switch the protocol to using remote URI's, rather than remote names.
  Names change. Also avoids issues with serialization of names containing
  whitespace.

This is nearly ready for merge into master now. I'd still like to make the ssh
transport smarter about reusing ssh connection caching during git pull.

This commit was sponsored by Jim Paris.

RemoteDaemon/Core.hs

@@ -106,7 +106,7 @@ genRemoteMap h@(TransportHandle g _) ochan =
 					ichan <- newChan :: IO (Chan Consumed)
 					return $ Just
 						( r
-						, (transport r (Git.repoDescribe r) h ichan ochan, ichan)
+						, (transport r (RemoteURI u) h ichan ochan, ichan)
 						)
 			_ -> return Nothing
 		_ -> return Nothing