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)

assigned to @gmoshkin

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marked this merge request as draft

mentioned in issue #370 (closed)

mentioned in issue #367 (closed)

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• 8e261a88...9f8f90f0 - 199 commits from branch master
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marked this merge request as ready

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• 3963fb5e - fix: forbid direct proposal of entries on followers

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• af852ff3 - fix: forbid direct proposal of entries on followers

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requested review from @rosik

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