Design a two step token migration workflow for the OCRv2 aggregator

[krebernisak]

How come we accept the token_mint as an input param when it's already available with state.config.token_mint, but then we don't accept token changes as required here?

Looks like the LINK token is not settable/upgradeable, right?

Originally posted by @krebernisak in https://github.com/smartcontractkit/chainlink-solana/pull/380#discussion_r968735664