Refactor ABSTAIN to UNSET

[Ramarti]

Reviewing AccessController logic, ABSTAIN confused me because of the connotations.
It's used as indicative of 0, no value set yet.
UNSET helped me understand better.

[kingster-will]

'ABSTAIN' is a term that is commonly utilized within permission systems.
For example: "The authorization .... makes the ultimate decision by checking the individual responses (permit, deny, abstain) "

[Ramarti]

I understand, but adapting it to Solidity development, it's more intuitive (at least to me) to understand that the value is a consequence of a value that's not set.
Abstain made me think the user did a conscious action to set that value, when most of the time the value is not something the user set, just Solidity returning 0 if a uint8 is not set.
We would probably not be naming that value if Solidity had null or undefined.

[Ramarti]

Just trying to improve code readability for audits.

[jdubpark]

• Many Solidity practices treat 0 as "unset"
• "Unset" is treated as abstain in the Access Controller

I think it's more apt to describe it closely to the Access system, ie. ABSTAIN. We could interchangeably use the terms ABSTAIN and UNSET since the former is what's used but the latter is a common theme in Solidity (value 0), just need to make it clear in the comments and documentation.

[jdubpark]

Closing as it seems like we've settled on "ABSTAIN." Please comment if there's misalignment @Ramarti