Refactor: respond to client even when leader is switched

[drmingdrmer]

Changelog

Refactor: respond to client even when leader is switched

Before this commit, the channels used for sending responses to the
client were dropped whenever a leader stepped down, which was not
necessary.

In the current commit, when a leader quits, we now preserve the channel, ensuring that:

• An OK response is sent to the client if the log entry the client has
  written is successfully committed;

• Alternatively, a ForwardToLeader error is sent if the log is
  reverted due to a conflict with the new leader.

In short, OpenRaft now makes a concerted effort to deliver a success
message to the client wherever possible.

Two test cases have been added to cover both successful and erroneous
scenarios.

Special thanks to @tvsfx for proposing this refinement :D

• Fix: Refactor: Move client_resp_channels from LeaderData to new tx_...-field in RaftCore #963

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This change is 

[drmingdrmer]

Reviewed 6 of 6 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @ariesdevil and @lichuang)

[tvsfx]

Reviewed 6 of 6 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @ariesdevil, @drmingdrmer, and @lichuang)

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openraft/src/core/raft_core.rs line 1582 at r1 (raw file):

                    AsyncRuntimeOf::<C>::spawn(async move {
                        for (log_index, tx) in removed.into_iter() {
                            let res = tx.send(Err(ClientWriteError::ForwardToLeader(ForwardToLeader {

This is actually a nice extra benefit of this approach that I did not think of; rather than having an empty ForwardToLeader error like before in QuitLeader, you can now redirect the client to the new leader immediately.