feat(server): Make DbSlice consumer an interface

[dranikpg]

1. Introduce ChangeConsumerInterface to allow for more interactions with registered consumers
2. Change latch to be only a guard to prevent removal during traversal of change callbacks
3. Use consumer interface to determine if serialization is going on and create proper functions for the db_slice
4. Create a serializer_base_test test that shows how eviction could have happened earlier before a delayed entry was written, effectively cancelling the eviction

[dranikpg]

augment review

[augmentcode]

🤖 Augment PR Summary

Summary: This PR refactors DbSlice change notification from a callback+id model to a consumer interface, enabling richer interactions with snapshot/serialization code.

Changes:

• Introduced DbSlice::ChangeConsumerInterface and converted registered change listeners from (version, callback) to consumer pointers.
• Replaced the previous serialization latch usage with a new change_cb_latch_ intended to guard safe traversal/removal of registered consumers.
• Added DbSlice helpers (WillBlockOnJournalWrite, WaitForUnblockedJournalWrites) that query consumers for blocked-bucket state to avoid journal-write preemption during sensitive traversals.
• Updated cluster slot flushing to register a temporary consumer object (via a CallbackConsumer) and unregister it after the fiber finishes.
• Updated SerializerBase to implement the new consumer interface and report/await bucket dependency blocking via BucketDependencies.
• Updated tests to use the new consumer API and added serializer regression coverage around delayed-entry eviction/expiry interactions.

Technical Notes: Snapshot versions are now stored on the consumer object itself (snapshot_version_), and the slice consults consumers to decide whether journaling can safely proceed without being rejected by blocked serialization.

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[augmentcode]

Review completed. 4 suggestions posted.

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[augmentcode]

src/server/db_slice.h:89: ChangeConsumerInterface is used as a polymorphic base, but it doesn’t declare a virtual destructor, which can cause UB if a consumer is ever deleted through a ChangeConsumerInterface* (easy to do with interfaces). Consider adding a virtual default destructor to make ownership patterns safer.

Severity: low

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[augmentcode]

src/server/serializer_base_test.cc:415: In DelayedAllDeleted, the loop after advancing time uses SET instead of asserting the keys were actually expired/deleted, so the test may no longer exercise the “bucket becomes empty due to expiry deletion” scenario described in the header comment. If the goal is to validate that expiry-driven deletions flush delayed reads, it may be worth reintroducing an explicit assertion/trigger of expiry deletion here.

Severity: medium

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