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 It has been replaced
by a call to `forceUpdateMetamaskState`.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.
