Deduplicate points for shard reads

[timvisee]

Tracked in: #4213

Deduplicate points based on their point ID in all reads from shards.

This is necessary for resharding, where multiple shards may have a single point ID.

The idea was to only deduplicate this while resharding is active, but after a discussion with @generall we decided to enable it at all times. We might benefit from this in other cases too, such as when we have the same point ID across multiple shard keys. At this time it is unclear yet what performance impact that might have, so that's yet to be tested.

Tasks

• Add tests for every deduplication scenario

Open questions

• Do we need to handle read errors in any special way?
  • e.g., ignore read errors from new shard?

All Submissions:

• Contributions should target the dev branch. Did you create your branch from dev?
• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

New Feature Submissions:

1. Does your submission pass tests?
2. Have you formatted your code locally using cargo +nightly fmt --all command prior to submission?
3. Have you checked your code using cargo clippy --all --all-features command?

Changes to Core Features:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your core changes, as applicable?
• Have you successfully ran tests with your changes locally?

[coszio]

I assume there is no way to dedup in count since the response from each shard is just a number

[ffuugoo]

LGTM. I like how simple the implementation turned out to be. 👌

[timvisee]

@coszio is right that count should be updated too. I propose to do that in a separate PR though, as it requires a different approach than what I've done in this PR.