Have MessageViewer use a custom Akonadi::Session instead of the
shared default one. This way the MessageViewer will still be able
to show message, even if the default session is currently busy.
This addresses the issue when a long-running operations (like large
moves) block the default session, so that user has to stare at the
"Retrieving folder contents" screen until the operation finishes.

This is not as good as a generic "session pool" solution, but that
introduced various issues in behaviour and consumed more resources.