Check canServeReadsFor in getMores

The "getMore" command path appears to never call canServeReadsFor, meaning that we could end up serving reads during a rollback. Data in this time could be inconsistent. For rollback via refetch, we refetch documents from the future and then play updates on them from the past which can lead to documents that never existed being returned. Rollback to a Timestamp is likely safe since it never is in an inconsistent state, but it's worthwhile to be consistent with the "find" command here.

Discovered by david.storch.