Improve compressed WAL write throughput

[moonsphere]

Summary:

• Add a prepared WAL record path so compressed WAL write groups can precompress outside the single WAL emission path.
• For normal zstd-compressed WAL write groups, merge the WriteBatch once, split the merged logical WAL payload across group threads for zstd precompression, then emit one logical WAL record to preserve recovery semantics.
• Add WAL precompression stats/perf context counters and coverage for prepared compressed WAL records and DB write group recovery.

Tests:

• make -j4 log_test db_write_test statistics_test perf_context_test
• ./log_test --gtest_filter='CompressionLogTest'
• ./db_write_test --gtest_filter='ParallelCompressedWALPrecompression'
• ./statistics_test
• ./perf_context_test --gtest_filter='PerfContext'

[meta-cla]

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