Before [1], the bus was started lazily: either as a consequence of
the addition of a channel to listen to or by manually calling the
`startPolling` method.
Before this commit, the websocket would have been started as soon as
the bus service starts which degrades performances.
This PR fixes the issue by re-introducing the same mechanism as before
that is by starting the websocket either by calling manually the `start`
method of the bus service or automatically when adding a channel.
[1]: odoo#75510
closesodoo/odoo#108741
X-original-commit: 800e175c5bc5d8ba23265c69c5c2cab6957fc73a
Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>