[WBE] Collect information from workers

* Added the `Notify` message type. Used to notify workers
  about executor has been started and to receive workers
  information back (topic and tasks list);
* Implemented `WorkersCache` to interact with workers
  information easly;
* Got rid of the engine `workers_info` parameter.

Change-Id: I4a810b1ddb0b04c11d12b47afc5f9cdf77d070be
Merge "[WBE] Collect information from workers"