feat(storage): introduce append batch

[ijsong]

What this PR does

There were already two kinds of batches in storage:

• The write batch saves only data keyed by the LLSN.
• The commit batch saves only the commit context and GLSN, and the commit context must exist.

This patch introduces a new batch type - append batch. The append batch saves both log entries and
a commit context. However, it is not necessary to keep the commit context. During synchronization,
the append batch must store log entries from a destination replica.

Which issue(s) this PR resolves

Updates #125

[codecov-commenter]

Codecov Report

Base: 62.91% // Head: 62.96% // Increases project coverage by +0.05% 🎉

Coverage data is based on head (0534ad0) compared to base (7b67ba2).
Patch coverage: 69.76% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #182      +/-   ##
==========================================
+ Coverage   62.91%   62.96%   +0.05%     
==========================================
  Files         126      127       +1     
  Lines       17178    17191      +13     
==========================================
+ Hits        10807    10824      +17     
- Misses       5815     5817       +2     
+ Partials      556      550       -6     

Impacted Files	Coverage Δ
internal/storage/testing.go	36.36% <0.00%> (+10.55%)	⬆️
internal/storage/append_batch.go	82.35% <82.35%> (ø)
internal/storage/storage.go	88.15% <100.00%> (+0.11%)	⬆️
internal/storagenode/reportcommit_server.go	70.51% <0.00%> (-7.67%)	⬇️
internal/metarepos/report_collector.go	76.42% <0.00%> (-1.70%)	⬇️
internal/storagenode/logstream/executor.go	80.20% <0.00%> (-1.21%)	⬇️
...metarepos/dummy_storagenode_client_factory_impl.go	63.90% <0.00%> (-0.72%)	⬇️
internal/metarepos/raft_metadata_repository.go	72.24% <0.00%> (+0.26%)	⬆️
internal/metarepos/raft.go	80.41% <0.00%> (+0.41%)	⬆️
internal/metarepos/storage.go	81.13% <0.00%> (+1.16%)	⬆️
... and 1 more

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