refactor(dav): put every method from OCA\DAV\CalDAV\CalDAVBackend tha…

…t does multiple DB calls in transactions

In a lot of methods we're doing read-after-writes (for instance calling
updateProperties after touching calendar objects).
There's also a lot of deleting methods that do stuff sequentially which
could cause trouble.
This should avoid this kind of issues.

Signed-off-by: Thomas Citharel <tcit@tcit.fr>