Implement task cancellation as a classmethod in each runner.

[warownia1]

• make scheduler respond to the events sent by the server (maybe runners as well)
  • puts more workload onto the scheduler
  • any scheduler slowdown will impact server reply time
• add cancel flag requests in the database
  • requires additional data synchronization between the database and scheduler
  • the in-memory list of running jobs may need to be removed and fetched directly from the database