[kv] Batch old-value lookups in the KV write path #4063

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Motivation

When processing a KV write batch, KvTablet may need to read the existing value of each key to generate the correct merged value and CDC records.

Currently, the write path checks the pre-write buffer first and then performs an individual RocksDB point lookup for every missing key. For a batch containing N distinct keys whose values are stored in RocksDB, this can result in N RocksDB JNI calls.

These lookups are executed while the tablet write lock is held. The repeated JNI crossings and serialized point lookups therefore increase the lock-holding time and limit write throughput, especially for large partial-update batches and tables using the FULL changelog image.

Solution

atch the RocksDB old-value lookups performed while processing one KV write batch:

• Decode the incoming records and identify the keys that require an old value.
• Check the pre-write buffer first because it contains values newer than RocksDB.
• Preserve pre-write tombstones without falling back to RocksDB.
• Deduplicate the remaining keys and load them with one RocksDB multiGet.
• Continue processing records in their original order.
• Recheck the pre-write buffer before using a prefetched RocksDB value so that repeated keys in the same batch observe earlier mutations from that batch.
• Keep the existing fast path for WAL changelog writes that do not require old values.

This optimization should reduce RocksDB JNI crossings for old-value lookup from up to N point-get calls to at most one multi-get call per KV record batch.

The change must not alter:

• KV tablet locking semantics.
• Pre-write buffer visibility.
• Record processing order.
• WAL and CDC ordering.
• Partial-update, aggregation, versioned, first-row, or auto-increment semantics.
• Duplicate batch detection and rollback behavior.
• Flush, snapshot, recovery, or lifecycle behavior.

Anything else?

No response

Willingness to contribute

• I'm willing to submit a PR!