[Clock dep] Two Party POL holder - Remove Clock dependency

[stiiifff]

As implemented in #240, implement the concept of privileged accounts for either permissioned or permissionless ticking operation of the contract.

• [ ] Remove dependency to Clock contract, and the Enqueue message at instantiation
• [ ] Add Privileged Accounts, and a guard for the ticking operation
• [ ] Update top-level covenants (using this contract) to set its address in the Clock's initial_queue
• [ ] Update / add tests as needed