Ensure autocommit is always activated.

Ensure autocommit is always activated.

This should have two major consequences:
* the rpc should have a view of the db consistent with the view of the
website
* this should reduce the overall memory footprint of the mysql database
since there is no "in progress transaction" and thus no need to keep old
data in the db.

Signed-off-by: Florent Bruneau <florent.bruneau@polytechnique.org>