The CurrencyRateController has been migrated to the new base
controller. The configuration can now only be set during construction,
as it was never changed at runtime in practice with the old controller.
Similarly, the `disable` function was removed as it wasn't relied upon
in practice.

One major change is that the controller doesn't poll after being
constructed. The "start" method must be called for polling to start.
The "start" and "stop" methods have been added to loosely follow the
conventions used in `metamask-extension` to allow starting and stopping
polling for the purpose of reducing network traffic.

A few tests required substantial updates because of this change. The
ComposableController tests were easiest to fix by deleting the use of
the CurrencyRateController completely, since that test was intended to
test BaseController-based controllers specifically.

The last TokenRatesController test managed to mark the state change
handlers in that controller as 'covered' without actually asserting
anything related to that functionality. That broken test was split into
two pieces, one for each state change handler. Those two tests still
rely upon mocking a would-be private function, which isn't great, but
it matches the patterns used elsewhere in that test module. To improve
it would take a great deal of work, enough for a separate PR.

The `handle` private variable has been renamed to better reflect what
it is. Also the type was updated to the more-specific `NodeJS.Timeout`.

It seems a bit odd to call this a "timeout" given that it's for a
repeating interval, but this is the term the Node.js docs use, and I
can't think of a better one.

They've been renamed to match the terms used in the MDN docs.