This change introduces a `nonceFn` field that can be changed
by a test but at runtime is transparent to the code that calls it.
The current nonce-generating logic defaults to a test-only field which
opens us up to this value inadvertently being set.
With this change, the default behavior becomes using `base62` to
generate a random nonce, and any tests have to explicitly change that.
With this change, the Worker is now aware of its own listeners in a
more codified and structured way.
These changes have already been made in the controller on
8d48a8515c. They're now made on the
worker for similar reasons.
Hugo Vieira