RaftActor shouldn't delete committed or non-conflict entries

[xirc]

Closes https://github.com/lerna-stack/akka-entity-replication/issues/152

There might be two places to be able to fix this bug:

• AppendedEntries replay or
• AppendEntries handling

This PR changes AppendEntries handling to fix this bug. pros:

• possible to maintain backward compatibility (changing event replay might break the existing Raft log)
• reduce required event storage space since RaftActor persists only new entries (including conflicts)

TODO

• [x] Fix main code
• [x] Write unit tests
  • [x] RaftMemberData.resolveNewLogEntries
  • [x] ReplicatedLog.findConflict
  • [x] ReplicatedLog.truncateAndAppend
  • [x] receiving AppendEntries
  • [x] RaftActor can read old AppenddedEntries event
• [x] Run one AZ failure gatling scenario (not included in this project) for verifying this bug fixed
• [x] Write an issue describing this bug
• [x] Write CHANGELOG

[xirc]

@negokaz As our team discussed, we have to fix this bug. Our team created a draft (not the exact same as this PR's code) fixing this bug and also verified this strategy might be the right direction. I've created a fix patch for this bug (is possible to merge but not unit-tested yet). Could you do a quick review of the code? I'm going to run the Gatling scenario and also write tests. I've already verified that existing tests pass.