Add a dedicated go-routine whose sole responsibility is to manage the
life time of the IMAP and SMTP servers and their listeners.
The current implementation behaves the same way as the previous state.
The new behavior will be implemented in a follow MR.
Depending on the timing of bridge closure, it was possible for the
IMAP/SMTP servers to not have started serving yet. By grouping this in
a cancelable goroutine group (*xsync.Group), we mitigate this issue.
Further, depending on internet disconnection timing during user login,
it was possible for a user to be improperly logged in. This change
fixes this and adds test coverage for it.
Lastly, depending on timing, certain background tasks (updates check,
connectivity ping) could be improperly started or closed. This change
groups them in the *xsync.Group as well to be closed properly.
There was a race condition internal to the go-smtp library.
In order to fix it, a version bump was necessary.
However, this significantly changed the library interface.
