Our request handlers do not always compute the query plan at the same time:
CqlRequestHandler waits until onThrottleReady(), and then passes the plan around as a method parameter
GraphRequestHandler waits until onThrottleReady(), and stores it in a field.
ContinuousRequestHandlerBase does it in the constructor.
Waiting for onThrottleReady is a bit more precise because we retrieve the plan when the request actually starts. That being said, I'm not sure if it's worth making that distinction, if we do it in the constructor we can store it in a final field which makes the code slightly simpler.