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
Handles all IMAP related tasks. Unlike the previous iteration, this new
service automatically adds and removes users from Gluon by interfacing
with server manager.
