* Ensure IMAP service sync cancel request waits until the sync has
completely cancelled rather than just signaling. It's possible that
due the context reset on `group.Cancel` that something may have not
have been bookmarked correctly in subsequent sync restarts.
* Handle connection lost/restored events in the services. Removes the
need to lock bridge users. Which could conflict with other ongoing
lock operations. Additionally, it ensure that if one service is
blocked it doesn't block the entire bridge.
* Revise access to bridge user locks.
