[Clock dep] Remote chain splitter - Remove Clock dependency

[stiiifff]

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

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