Refactor consistent hashing implementation

[shaqque]

Overview

This PR refactors the Jump consistent hashing implementation to allow using other consistent hashing algorithms. Shard distribution to nodes now happen on a per-index basis, which means different indexes can be distributed using different algorithms.

All pre-existing indexes will default to the current Jump algorithm, while new indexes will be allocated using the default shard distribution algorithm set when the server starts/restarts.

Fixes #2035

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• [x] All new and existing tests pass.
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[shaqque]

@jaffee the tests are failing because the protobuf files are not yet generated. Unfortunately, I have the latest version of protoc/protobuf on my computer which adds trash fields like XXX_blahblah. Otherwise, all tests pass locally on my computer with the generated protobuf files and this PR should be ready.

An older version of protoc/protobuf should generate the protobuf files without the trash fields.