perf(sn): reuse buffer for ReplicateRequest unmarshaling

[ijsong]

What this PR does

Improve unmarshaling performance by reusing buffers for ReplicateRequest in the
backup replica.

The protobuf message github.com/kakao/varlog/proto/snpb.(ReplicateRequest) has
two slice fields—LLSN ([]uint64) and Data ([][]byte). The backup replica
receives replicated log entries from the primary replica via the gRPC service
github.com/kakao/varlog/proto/snpb.(ReplicatorServer).Replicate, which sends
ReplicateRequest messages.

Upon receiving a ReplicateRequest, the backup replica unmarshals the message,
which involves growing slices for fields such as LLSN and Data. This growth
causes copy overhead whenever the slice capacities need to expand.

To address this, we introduce a new method, ResetReuse, for reusing slices
instead of resetting them completely. The ResetReuse method shrinks the slice
lengths while preserving their capacities, thus avoiding the overhead of
reallocating memory.

Example implementation:

type Message struct {
    Buffer []byte
    // Other fields
}

func (m *Message) Reset() {
    *m = Message{}
}

func (m *Message) ResetReuse() {
    s := m.Buffer[:0]
    *m = Message{}
    m.Buffer = s
}

Risks:

This approach has potential downsides. Since the heap space consumed by the
slices is not reclaimed, the storage node's memory consumption may increase.
Currently, there is no mechanism to shrink the heap usage.

Additionally, this PR changes the generated code. The protobuf compiler can
revert it, which is contrary to our intention. To catch this mistake, this PR
includes a unit test (github.com/kakao/varlog/proto/snpb.TestReplicateRequest)
to verify that the buffer backing the slices is reused.

Which issue(s) this PR resolves

Resolves: #795

See also: #806

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.44%. Comparing base (6a93655) to head (8ef1886).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #808      +/-   ##
==========================================
+ Coverage   68.09%   68.44%   +0.35%     
==========================================
  Files         182      182              
  Lines       17899    17903       +4     
==========================================
+ Hits        12188    12254      +66     
+ Misses       4941     4863      -78     
- Partials      770      786      +16     

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