storage: consistency checker diff calculation uses a cancelled context

[petermattis]

Discovered while investigating #10602.

When Replica.ConsistencyChecker discovers an inconsistency, it spawns a new goroutine using Stopper.RunAsyncTask and recursively calls itself pass withDiff = true. But the new goroutine is passed the original context and when Replica.ConsistencyChecker returns the context is cancelled fairly soon. The result is that we never perform the diff check and never exist on consistency failures.