sql/swh-scheduler-data: Fix some task names and add svn update task
This fixes some tasks names and backend names (related to Mercurial loading) in order to be synchronized with what is deployed in production.
I have also added a new task 'origin-update-svn', for loading svn repositories through the 'dump first, then mount and load' approach.
Migrated from D533 (view on Phabricator)