The `nodeify` utility is no longer needed for the background API.
Instead each method is assumed to be either synchronous or Promise-
returning.
The error handling was updated to at least log the error in the case
where a method fall fails after the connection is broken.
As part of the unlock process, currently the seed phrase is retrieved
from the background then discarded. This step is pointless, so it has
been deleted.
The CurrencyRateController has been migrated to the BaseControllerV2
API, which includes various API changes. These changes include:
* The constructor now expects to be passed a
`RestrictedControllerMessenger`.
* State changes are subscribed to via the `ControllerMessenger` now,
rather than via a `subscribe` function.
* The state and configration are passed in as one "options" object,
rather than as two separate parameters
* The polling needs to be started explicitly by calling `start`. It
can be stopped and started on-demand now as well.
* Changing the current currency or native currency will now throw an
error if we fail to update the conversion rate.
The `ComposableObservableStore` has been updated to accomodate these
new types of controllers. The constructor has been updated to use an
options bag pattern as well, to make the addition of the new required
`controllerMessenger` parameter a bit less unweildly.
The `SET_CURRENT_FIAT` action has been removed. It has been replaced
by a call to `forceUpdateMetamaskState`. The only purpose of this
action was to eagerly update the current fiat currency settings before
the next state update. Forcing a state update instead is simpler and
safer.
The `setCurrentCurrency` function in the background has been updated to
no longer return the state, now that it's no longer needed.
