Document QueryStateMachine in developer docs

[maxalbert]

The new QueryStateMachine class introduces in #434 plays a central part in the logic of queries are run and interact with redis and the database.

The key concepts of how this works should be documented in the developer docs. Also, in the comments on #434 @greenape added a state transition diagram which is extremely useful and shouldn't get lost:

graph LR
A[KNOWN] -->|ENQUEUE| B[QUEUED]
B -->|EXECUTE| C[EXECUTING]
C -->|ERROR| D[ERRORED]
C -->|FINISH| E[COMPLETED]
C -->|CANCEL| F[CANCELLED]
B -->|CANCEL| F[CANCELLED]
D -->|RESET| G[RESETTING]
E -->|RESET| G[RESETTING]
F -->|RESET| G[RESETTING]
G -->|FINISH_RESET| A
H[Blocking]
style B fill:#f9f
style C fill:#f9f
style G fill:#f9f
style H fill:#f9f