CollectionStatistics: prefetch all stats on start

When Akonadi is started each application and agent starts requesting
its Collections and statistics. This causes the CollectionStatistics
cache to query the stats for each Collection individually with the
very expensive query. On my system all the statistics queries together
take over 17 seconds.

With this change the CollectionStatistics cache simply pre-fetches
statistics for all Collections in single a query, which is MUCH
faster (less than a second on my system), because the expensive JOINs
only have to be done once.

As a result of this change applications and agents become responsive
much faster after start and CPU and IO load during start is massively
decreased.