HDDS-16229. Add RocksDB multiGetSkipCache for batched table reads.

[SaketaChalamchala]

What changes were proposed in this pull request?

RocksDB MultiGet support through RocksDatabase, RDBTable, and TypedTable on a skip-cache path. Callers can batch point lookups in one RocksDB call instead of looping get() / getSkipCache().
Added multiGet with CodecBuffer in TypedTable.

Missing keys return null at the matching list index; order matches the input key list.

Out of scope (follow-ups): cache-aware Table.multiGet, batch metrics, call-site adoption.

What is the link to the Apache JIRA

https://issues.apache.org/jira/browse/HDDS-16229

How was this patch tested?

Unit Tests

[swamirishi]

Thank you for the patch @SaketaChalamchala this is a good improvement to have.

[swamirishi]

Please implement CodeBuffer based multiGets as well otherwise the GC pressure on memory would be bad

https://github.com/facebook/rocksdb/blob/805ac7c887bbebd7e6124bbc7e8d0803aeed8f07/java/src/main/java/org/rocksdb/RocksDB.java#L2435-L2460

You would need to implement a similar function like

ozone/hadoop-hdds/framework/src/main/java/org/apache/hadoop/hdds/utils/db/TypedTable.java

Lines 318 to 355 in 9fa7fff

	private VALUE getFromTable(KEY key,
	CheckedBiFunction<CodecBuffer, CodecBuffer, Integer, RocksDatabaseException> get)
	throws RocksDatabaseException, CodecException {
	try (CodecBuffer inKey = keyCodec.toDirectCodecBuffer(key)) {
	for (; ;) {
	final Integer required;
	final int initial = -bufferCapacity.get(); // resizable
	try (CodecBuffer outValue = CodecBuffer.allocateDirect(initial)) {
	required = get.apply(inKey, outValue);
	if (required == null) {
	// key not found
	return null;
	} else if (required < 0) {
	throw new IllegalStateException("required = " + required + " < 0");
	}
	for (; ;) {
	if (required == outValue.readableBytes()) {
	// buffer size is big enough
	return valueCodec.fromCodecBuffer(outValue);
	}
	// buffer size too small, try increasing the capacity.
	if (!outValue.setCapacity(required)) {
	break;
	}
	// retry with the new capacity
	outValue.clear();
	final int retried = get.apply(inKey, outValue);
	Preconditions.assertSame(required.intValue(), retried, "required");
	}
	}
	// buffer size too small, reallocate a new buffer.
	bufferCapacity.increase(required);
	}
	}
	}

to ensure the value which didn't fit in the first multiget should be called again with a resized CodecBuffer size.
So the first call should do a multiGet with all CodecBuffer keys and based on the status list returned we need to call multiget again for the ones which didn't fit in the initial fit and collate the list.