To allow for finders to be shared (across executors) in
the near future we need to avoid assigning our callback
as the sole/single callback that the finder will use and
instead use a notification mechanism (using our notifier
type) that can be used to register many callbacks and
decouple the emitting of new workers events from the
reception of that event.
Part of blueprint wbe-worker-info
Change-Id: I0ab2ec649f759ec67e15960bdeb83b108174734b
f2a6acaff0