Perf: Optimize funtion DBImpl::Write in db/db_impl/db_impl_write.cc

[fyrsta7]

Summary

This PR optimizes the performance of the function DBImpl::Write in db/db_impl/db_impl_write.cc.
Performance was measured using the project's standard benchmark tools. Out of 2 test cases, the optimization achieves a maximum improvement of 5.04% while guaranteeing no regression exceeds 0.23% in any other case.

Test Plan

1. Correctness: All existing unit tests pass.
2. Performance: Evaluation was done using the following benchmark command: ./db_bench --benchmarks=fillseq,readrandom --db=/tmp/rocksdb_test --num=1000000

Performance Evaluation & Results

Testing Protocol:

• The benchmark was run on an isolated Ubuntu 24.04 server.
• The first run was discarded to account for cold-start effects.
• The results below are the average of 5 subsequent runs.
• The improvement for a test case is calculated as (new_value - old_value) / old_value * 100% if a higher value is better (e.g., throughput), or (old_value - new_value) / new_value * 100% if a lower value is better (e.g., latency). A positive percentage indicates a performance gain.

Results:

Test Case	Improvement
readrandom_mb_change	-0.23%
fillseq_mb_change	5.04%

[fyrsta7]

Hi @pdillinger , could you please take a look at this PR when you have a moment? Thanks!

[fyrsta7]

Could a maintainer please approve the workflow to run? Thanks!