refactor: simplify raft entry proposal

- remove logical clock from raft entry context, (term, index) is used
  for synchronization instead
- this allows us to remove the Notify/Notifier boilerplate
- NodeImpl::notifications -> NodeImpl::read_state_wakers
- this and the recently refactored proc_join finally allows us to remove
  OpResult
- ContextCoercion & CoercionError are also removed for the sake of
  simplification