Rather than having the services subscribe to each individual event type,
have only one subscriber for the whole event object. This spreads the
logic to the service through the `EventHandler` type.
This is important for the next stage where the IMAP service will be
required to apply events from the past.
This patch adds the User Event Service which is meant to replace the
current event polling flow.
Each user interested in receiving events should register a new
subscriber using the `Service.Subscribe` function and then react on
the incoming events.
The current patch does not hook this up Bridge user as there are no
existing consumers, but it does provide extensive testing for the
expected behavior.
