feat(sharding): introduce config variable bucket_count for tiers

Summary

• feat(sharding): introduce config variable bucket_count for tiers

Signed-off-by: Anatoly Popov a.popov@picodata.io

Variable is available only in config.

You can specify it for exact tier:

cluster:
    tier:
        bucket_count: 20000

You also can change cluster default, which is 3000:

cluster:
    default_bucket_count: 60000

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• Close #1370 (closed)
• Cherry-pick to: none
• Docs follow-up: docs#465 (closed)
• Changelog is updated