In examine_updates() which is periodically executed in the IMAP watch
thread, the mailbox's contents get fetched regardless if the user has
fetched the mailbox before. That means eg a large mailbox that was
unused by the user might perform a large fetch all of a sudden without
the user's knowledge/touch.
Add `warm` property in ImapMailbox that states whether the mailbox has
been loaded before in current execution.
Closes#88 IMAP: don't examine unloaded mailboxes for updates, just for message count stats
Manos Pitsidianakis