Tweak TB_EMPTY event based on comments by nickm.

- Avoid control_event_refill_global function with 13 arguments and
  increase code reuse factor by moving more code from control.c to
  connection.c.
- Avoid an unsafe uint32_t -> int cast.
- Add TestingEnableTbEmptyEvent option.
- Prepare functions for testing.
- Rename a few functions and improve documentation.