Cope better with Akonadi failing to start

When Akonadi fails to start, we can detect it by watching for
org.freedesktop.Akonadi.Control.Lock service disappearing from DBus,
which means that akonadi_control has terminated and thus something
went wrong.

As a next step, we must abort all pending jobs, otherwise client
applications waiting for the job to finish will get stuck forever,
because the job will be waiting in Session for reply from the server.

REVIEW: 117144