[delta-audit] treasury: Avoid overriding vote of non self-bonding accounts

[victorges]

What does this pull request do? Explain your changes. (required) This fixes a finding from the audit regarding on overriding votes. It addresses the problem of delegating to a non-transcoder account and then overriding their votes imprperly.

The known corner cases that can trigger this inconsistency are:

• delegating to an account that isn't a transcoder (either another delegator or a non participant)
• transcoder unbonding their full amount (delegators stay delegated to it)

Specific updates (required)

• Update GovernorCountingOverridable to not override votes of accounts that don't self-delegate

How did you test each of these updates (required) yarn test to be deployed on devnet

Does this pull request close any open issues? Fixes:

• https://github.com/code-423n4/2023-08-livepeer-findings/issues/96
• https://github.com/code-423n4/2023-08-livepeer-findings/issues/206

Checklist:

• [x] README and other documentation updated
• [x] All tests using yarn test pass