+ Rename to await and yield.
~ Minor fixes.

* Annotate all required fields as strict. These are always used and
  there is no reason to keep them lazy.
* Augment user errors with location.
~ Fix bugs in tracker communication.

* Fix tracker interval timeout: threadDelay use argument is microseconds
  while tracker response is seconds.

* Fix stopped request: tracker might response with empty body.
- Remove some debug stuff.