Implement an offline mode for a distributed node (#4646)

Implement an offline mode for remote storage to cache the
offline status of a node in order to prevent network calls
that are bound to fail. After a time interval an attempt
will be made to restore the connection and mark the node
as online if successful.

Fixes #4183

cmd/storage-rpc-client.go

@@ -45,11 +45,11 @@ func toStorageErr(err error) error {
 
 	switch err.(type) {
 	case *net.OpError:
-		return errDiskNotFound
+		return errDiskNotFoundFromNetError
 	}
 
 	if err == rpc.ErrShutdown {
-		return errDiskNotFound
+		return errDiskNotFoundFromRPCShutdown
 	}
 
 	switch err.Error() {