Adjust readonly field generation logic

[wanglin86769]

1. Use nullable Boolean class instead of primitive boolean for last_readonly and readonly, so that the first value does not equal to null and "readonly" field can show up in WebSocket response.
2. Ensure both last_readonly and readonly are the OR logic of isReadonly() and !PV_WRITE_SUPPORT, because they will be compared in toJson() method.

[kasemir]

Looks good.

Are you actually enabling write access in your setup?
How do you control who is allowed to write?

[wanglin86769]

No, we are not enabling write access.
Our use case at CSNS is using pvws to access PVs at office network via a CA gateway which provides readonly permission and publishes PVs from the control network.