[bfss] fix file update to ensure file's content is available on the fs asap... and not only at commit time. So it's consistent with entity creation behaviour. The new file is created at assignement time and removed if the commit is rollbacked.

addressbook
basket
blog
book
calendar
comment
company
email
file
folder
i18ncontent
keyword
link
mailinglist
person
tag
timeseries
vcsfile