Add storage access restriction at method level

[lock9]

Suggestion:
Add additional storage access restriction at method level.

We can potentially increase TPS by running stateless methods concurrently.

Maybe the method could have 3 execution levels:
Stateless
ReadOnly
ReadWrite

I know about the need for the stateless methods (#822), but I'm not very confident if we need ReadOnly and ReadWrite. Maybe we just need Stateless and Stateful types.

[igormcoelho]

By method, you mean CALL, right? Or APPCALL?
The only issue in executing things in parallel is that you need to sync execution at some point...

I mean, you execute ContractA (or MethodA) and now you execute ContractB (or MethodB). When will you stop waiting?

[shargon]

or in an attribute inside transaction?

[lock9]

We can leave this stateless processing to be done "before or after" the stateful part is done.
Example:

We should allow only stateless methods in pre and post-processing stages.

This is not mandatory to NEO 3.0, it can be added later.

[lock9]

Closing this in favor of #919

[igormcoelho]

I think this is more connected to: #927

Is it different than 927?