reimplement format conversion for file-sync

[bricks]

In r3416 the internal format conversion was removed. This needs to be reimplemented - or handled inside OpenSync?.

[felixmoeller]

move back to current release ;)

[prahal]

Fixed in http://www.opensync.org/changeset/3659 . Now plugins could use <Preferred>objformat_name</Preferred> in <Formats> (in <Resource>). For example for file-sync use vcard30. Beware there is no check on previous preferred format so one has to clear the directory and run a slow-sync on the sink after changing the preferred format. We won't support mixing diferrent objformat in the same member . Though one could open a ticket so we check the current data match the format we'll use to prevent mistakes ...