Describe the bug
ReadStates and WriteStates are currently different flags and the latter doesn't imply the former. I think this is a bit misleading, because in most cases we use WriteStates read is also needed. This can lead to behavior described here nspcc-dev/neo-go#1725 .

Solution
I can think of two of them:

1. Make WriteStates require ReadStates (rename States to WriteStates). States is currently not used, so I think it makes sense.
2. Replace WriteStates with States in Transfer and possibly some other methods with callbacks (deploy and update come to mind immediately).

I prefer first solution.

To Reproduce
Try to read storage in onPayment method, like here nspcc-dev/neo-go#1725

Expected behavior
onPayment executes correctly.