Implement wpool:broadcall/2 function

[harenson]

Enable worker_pool to perform a synchronous call to every worker within the given pool, wait for the responses and send them back to the caller, e.g:

-spec broadcall(name(), Call) -> [response()].
broadcall(Sup, Call) ->
  Workers = all_workers(Sup),
  [wpool_process:call(Worker, Call, Timeout) || Worker <- Workers].