refactor: improve append entries

[GG2002]

Please briefly answer these questions:

• what problem are you trying to solve? (or if there's no problem, what's the motivation for this change?) Optimize the behavior of log append entries. Ref to #515
• what changes does this pull request make? The behavior of log append entries was redudant and this pr has made optimization according to following steps:
  1. Find the index of the first entry that needs to be truncated.
  2. Record entries that need to be fallback in the truncated entries.
  3. truncate entries
  4. Push the remaining entries and record the conf change entries
• are there any non-obvious implications of these changes? (does it break compatibility with previous versions, etc) No.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 80.00000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 74.44%. Comparing base (5fce2c6) to head (6bbef57). Report is 3 commits behind head on master.

Files	Patch %	Lines
crates/curp/src/server/raw_curp/log.rs	80.00%	2 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #663 +/- ## ========================================== + Coverage 74.39% 74.44% +0.04% ========================================== Files 170 170 Lines 24827 24836 +9 Branches 24827 24836 +9 ========================================== + Hits 18470 18489 +19 + Misses 5211 5201 -10 Partials 1146 1146 ```

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