Update to latest Gluon to allow access to the database for bridge. The
cache is placed in a `SharedCache` type to ensure that we respect calls
to `Connector.Close`.
In theory, this should never trigger an error, but this way we can catch
it if it happens.
https://github.com/ProtonMail/gluon/pull/391
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.
