Error transitioning worker task from `waiting` to `fetch`

Similar to https://github.com/dask/distributed/issues/4721, I had some folks report offline that they were encountering an error due to an unsupported task state transition on their workers (traceback shown below)

```python
Traceback (most recent call last):
File "/usr/local/python/python-3.7/std/lib64/python3.7/site-packages/tornado/ioloop.py", line 741, in _run_callback
ret = callback()
File "/usr/local/python/python-3.7/std/lib64/python3.7/site-packages/tornado/ioloop.py", line 765, in _discard_future_result
future.result()
File "/usr/local/python/python-3.7/std/lib64/python3.7/site-packages/distributed/worker.py", line 2261, in gather_dep
self.transition(ts, "fetch", worker=worker)
File "/usr/local/python/python-3.7/std/lib64/python3.7/site-packages/distributed/worker.py", line 1584, in transition
func = self._transitions[start, finish]
KeyError: ('waiting', 'fetch')
```

which was observed using `dask` and `distributed` 2021.04.0

cc @fjetter @gforsyth