Implement CompareAndSetBlockData using offchain storage

[timwu20]

Issue Summary

Taken from design doc:

CompareAndSetBlockData is currently used in sync and is already introduced in offchain.OffchainStorage.CompareAndSet(). It is implemented by offchain.LocalStorage which is a dependency of db.Backend. Adding the CompareAndSetBlockData function to the translation shim should therefore be relatively simple given the functionality is already implemented.

Acceptance Criteria

• Implement CompareAndSetBlockData in translation shim type introduced in Introduce a translation shim type that implements revised BlockState and StorageState backed by Client #4460
• Unit tests