Converting doesn't work in ZEO cluster with non-shared blobs

[lewicki]

There is a problem with converting of documents in ZEO cluster when blobs are not shared. Do I have a wrong setup or it is just a bug? It is far better in async setup when I configure the worker to share blobs with ZEO server, but I still had to patch 'can_convert' method as it is checking blobs even if job is going to be in a queue.

Original traceback:
Module collective.documentviewer.subscribers, line 36, in handle_file_creation
Module collective.documentviewer.async, line 139, in queueJob
Module collective.documentviewer.convert, line 379, in can_convert
Module collective.documentviewer.convert, line 366, in initialize_filehash
Module collective.documentviewer.convert, line 160, in get
Module collective.documentviewer.convert, line 120, in _run_command
Exception: Command /usr/bin/md5sum /my_zeo_client/var/blobstorage/972/46.03a974a129334c11.blob finished with return code 1 and output: /usr/bin/md5sum: /my_zeo_client/var/blobstorage/972/46.03a974a129334c11.blob: No such file or directory