Add "ready->memory" to transitions in worker

This should remove the error message in dask#4721 but is more of a band-aid
than a fix.  Ready->memory transitions shouldn't happen, but since they
do occassionally crop up, we might as well dispatch them appropriately
until the worker state machine is ready.

distributed/worker.py

@@ -466,6 +466,8 @@ def __init__(
             ("executing", "memory"): self.transition_executing_done,
             ("flight", "memory"): self.transition_flight_memory,
             ("flight", "fetch"): self.transition_flight_fetch,
+            # Shouldn't be a valid transition but happens nonetheless
+            ("ready", "memory"): self.transition_ready_memory,
             # Scheduler intercession (re-assignment)
             ("fetch", "waiting"): self.transition_fetch_waiting,
             ("flight", "waiting"): self.transition_flight_waiting,