if leader changed during raft::appendLog() RPC, use a different error code

[liuyu85cn]

What type of PR is this?

• bug
• feature
• enhancement

What problem(s) does this PR solve?

Issue(s) number:

Description:

It is nature to return E_RAFT_TERM_OUT_OF_DATE or LEADER_CHANGE
when raft check leader, but this will make caller believe the append failed.

however, this log is replicated to followers(written in follower's WAL)
and the follower may become leader (use that WAL)
which means this log may eventually commit succeeded.

How do you solve it?

Special notes for your reviewer, ex. impact of this fix, design document, etc:

Checklist:

Tests:

• Unit test(positive and negative cases)
• Function test
• Performance test
• N/A

Affects:

• Documentation affected (Please add the label if documentation needs to be modified.)
• Incompatibility (If it breaks the compatibility, please describe it and add the label.）
• If it's needed to cherry-pick (If cherry-pick to some branches is required, please label the destination version(s).)
• Performance impacted: Consumes more CPU/Memory

Release notes:

n/a

[panda-sheep]

Good！

[critical27]

LGTM