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.
Identity Service contains all the information related to user state,
addresses and keys.
This patch also introduces the `State` type which can be used by other
services to maintain their own copy of this state to avoid lock
contention.
Finally, there are currently no external facing methods via a CPC
interface. Those will added as needed once the refactoring of the
architecture is complete.
