* store the S3 uri in the 'store_fname' (s3://bucket/key)
* the read-only mode is now built-in, as we store the bucket name, if a
instance is started with a different bucket or another filestore
method, it will continue to read the previous attachments on their
stored bucket, but new attachments will be stored on the new one
* remove config in ir.config_parameter, it makes all the stuff more
complex and we don't use them (config file would be more interesting)
* automatically migrate the attachments on loading of the server, so
if an ir.attachment has been created during the module
upgrade/initialization before attachment_s3 is loaded, it will be sent
to S3 as soon as it's loaded
Guewen Baconnier