HDDS-16257. Avoid the flatten-copy in OmMetadataManagerImpl.getExpiredOpenKeys' hsync branch

[shuan1026]

What changes were proposed in this pull request?

OmMetadataManagerImpl.getExpiredOpenKeys's hsync-commit branch called OmKeyLocationInfoGroup.createLocationList() on the key's latest version group just to stream it once while building the commit KeyArgs protobuf, allocating a throwaway flattened List per matched hsync key.

This PR swaps the method reference for the zero-copy getLocationLists() accessor and inserts a flatMap(List::stream) to keep the rest of the chain (.map(getProtobuf).forEach(addKeyLocations)) unchanged.

Additionally, a throwaway (not committed) JUnit micro-benchmark measured per-KeyArgs-construction thread allocation for both implementations (JDK 21.0.12, 50,000-iteration warmup then 500,000 measured iterations per shape, output KeyArgs protobufs asserted equal, volatile sink to block JIT elimination):

Key Shape	Before	After	Saved
1 version list x 1 block	352 ns / 1372 B	177 ns / 1154 B	218 B
1 version list x 4 blocks	367 ns / 2496 B	387 ns / 2296 B	200 B
1 version list x 32 blocks	2155 ns / 13264 B	2086 ns / 12728 B	536 B
4 version lists x 4 blocks	1144 ns / 7432 B	1117 ns / 7048 B	384 B
16 version lists x 8 blocks	8176 ns / 51280 B	8005 ns / 49088 B	2192 B

What is the link to the Apache JIRA

https://issues.apache.org/jira/browse/HDDS-16257

How was this patch tested?

• TestOpenKeyCleanupService#testCommitExpiredHsyncKeys
• TestOpenKeyCleanupService#testCleanupExpiredOpenKeys
• TestOpenKeyCleanupService#testIgnoreExpiredRecoverhsyncKeys

[sarvekshayr]

Thanks @shuan1026 for working on this.

Minor nits not related to your changes.

[sarvekshayr]

nit: Add the import for Optional at the top.

Suggested change

	java.util.Optional.ofNullable(info.getLatestVersionLocations())
	Optional.ofNullable(info.getLatestVersionLocations())

[sarvekshayr]

Here as well.

[sravani-revuri]

agree with @sarvekshayr on the import. Otherwise LGTM.

[shuan1026]

Done, thanks @sarvekshayr @sravani-revuri for the review!