Use separate akonadi session for search jobs by default

e.g. those created while sending an email (for crypto preferences,
and possibly alias and distr list lookups).

This fixes kmail waiting for a possibly long-running search job,
queuing up any other akonadi job meanwhile, making it impossible
to read email.

The search jobs created by the completion are still using a different session,
because it also uses that session for RecursiveItemFetchJob and CollectionFetchJob.