Labels can be held locally and updated in memory. This greatly improves
the responsiveness of IMAP mailbox operations as we don't need to fetch
all a user's labels to find the parent whenever a mailbox is moved.
When changing address mode, we would close all a user's update channels
and create them from scratch. This involved setting user.updateCh to a
new value. However, it was possible for other goroutines to read from
user.updateCh during this time. I replaced it with a call to
user.updateCh.Clear(), which is threadsafe.
This fixes various race conditions and leaks related to the user's sync
and API event stream. It was possible for a sync/stream to begin after a
user was already closed; this change prevents that by managing the
goroutines related to sync/stream within cancellable groups.
