Fixes IMAP error causing all remote folders to be considered deleted
Better ClientSession handling in Imap.Account

Simplifies handling connection drops, which occur more frequently
when AUTHORIZED rather than SELECTED/EXAMINED w/ IDLE.
Fix regression where folders were not regularly updated w/ STATUS info

Now that Imap.Folders are being preserved, need to update their
FolderProperties w/ STATUS results whenever listed.
Prevent reconnect loop from occurring

I noticed recently that when composing a draft message, if the
connection to the Drafts folder was dropped by the server, Geary
would enter a fast reconnect loop.  The problem was that, even if
Imap.Folder signals "disconnect", it's close_async() must be called.
This adds that logic to the background reestablishment code.