refactor: simplify raft entry proposal

Summary

• 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

• fix: forbid direct proposal of entries on followers

  • removed method NodeImpl::propose (propose_async replaces it)
  • make NodeImpl::propose_async return Error on followers, only leader can propose entries directly, others should use CAS
  • remove all tests which used direct entry proposal on followers, they're not needed anymore

• Close #367 (closed)
• Close #309 (closed)
• Close #370 (closed)
• Close #195 (closed)