chore: minor refactor on etcd kvbackend

[fengjiachun]

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Refer to a related PR or issue link (optional)

What's changed and what's your intention?

As the title said:

1. Avoid clone in txn
2. Use KeyValue::from and remove convert_key_value method
3. Use KeyValue::from instead of Into::into for clearer code

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR does not require documentation updates.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.35%. Comparing base (4ca7ac7) to head (919a56f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3940      +/-   ##
==========================================
- Coverage   85.65%   85.35%   -0.30%     
==========================================
  Files         967      967              
  Lines      165999   165989      -10     
==========================================
- Hits       142184   141685     -499     
- Misses      23815    24304     +489