Remove sleep from pull-based event loop

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**

The current implementation sleeps (currently ~100ms) whenever there is no job active. This adds some extra latency to the jobs.
We could instead have the scheduler send a message to the executor that a new tasks are available.

**Describe the solution you'd like**
have the scheduler send a message to the executor that tasks are available.
remove sleeping logic from `poll_loop` (e.g. implement with mutex)

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

**Additional context**
Add any other context or screenshots about the feature request here.

Start by locating the executor's `poll_loop` and the scheduler-to-executor task-availability path. Determine how that communication can replace the current sleep; done means the executor no longer sleeps when idle and newly available tasks are handled without the added latency.

Written by the indexing model from the issue text.