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 moves the `user.User.CheckAuth` function into the `State`
type as it contains all the necessary information to perform this check.
A couple of more interfaces are added to abstract the retrieval of the
Bridge and the User's key passwords, as well as telemetry reports.
Finally, adds `State.OnAddressEvents` which other services should use to
update the state.
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.
