* Compute the content checksum (in addition to the optional
transmission checksum) during upload (.eml files only)
* Add hook to compute and compare the checksum in csync_update
* Add content checksum to database, remove transmission checksum
On unix we don't safe much (otherwise csync would have been
designed differently).
On windows however, the readdir already fetch all the info, so we
can as well use it.
We still have to query for the file id but we might optimize that later
We fetch the id from the server, but don't save them in the database.
I Could have used INSTRUCTION_UPDATED for that, but then i would need to update the
reconcile algorithm to take in account the fact that UPDATED is possible there.
Instead, use should_update_etag which means the db is going to be written again
Remove reference to old instruction _UPDATED and _DELETED which does not make sens with
the new propagator
Improve the test to test this case, and that etags are properly writen to the DB
when there is a fake conflict
