Fix bug #268986 empty trash when we close kmail.

When we close kmail it deletes foldercollectionmonitor and expunge is async
=> it deleted before to remove mail in trash
=> make it sync so it delete before to close foldercollectionmonitor.
There is still a bug, statistic count is not update when we move mail
in trash don't know why, perhaps it's necessary to update statistic
before to test if we must clean trash