Improve membership management

Changed:

• Changed: 1bd22edc remove AddLearnerError::Exists, which is not actually used; by 张炎泼; 2022-09-30

• Changed: c6fe29d4 change-membership does not return error when replication lags; by 张炎泼; 2022-10-22

  If blocking is true, Raft::change_membership(..., blocking) will
  block until repliication to new nodes become upto date.
  But it won't return an error when proposing change-membership log.

  • Change: remove two errors: LearnerIsLagging and LearnerNotFound.

  • Fix: #581

Fixed:

• Fixed: 2896b98e changing membership should not remove replication to all learners; by 张炎泼; 2022-09-30

  When changing membership, replications to the learners(non-voters) that
  are not added as voter should be kept.

  E.g.: with a cluster of voters {0} and learners {1, 2, 3}, changing
  membership to {0, 1, 2} should not remove replication to node 3.

  Only replications to removed members should be removed.

Added:

• Added: 9a22bb03 add rocks-store as a RaftStorage implementation based on rocks-db; by 张炎泼; 2023-02-22