Review the use of READ_ONLY_FLAG in cases we should use it

[emmanuelm41]

Whenever we tried to use READ_ONLY_FLAG in methods that should be read only, we broke CI tests. Let's check what is happening this flag. We should use it in cases the state is not going to be changed.

🔗 zboto Link

[emmanuelm41]

We are currently asking filecoin about this flag.

[emmanuelm41]

Reopening issue because we still need to make some methods read only. Both ones cannot be used as read only because an issue on the builtin actors code. Need to update the submodule to a newer version after the issue is fixed.

Miner --> getBeneficiary
VerifReg --> getClaims

[emmanuelm41]

Update being applied here #209