Improve update procedure

[capital-G]

• Only run if latest commit in db != current commit (add a flag to force an update nonetheless)
• Delete all docs of a project before updating - this avoids the creation of orphans (e.g. foo.schelp was renamed to foo2.schelp)
• Do the same for classes
• Add a report on what failed (e.g. which repos offline) - this could maybe be managed by a better logging mechanism