feat: make uuids truly random

Summary

• feat: make uuids truly unique

  Up to this patch instance_uuid and replicaset_uuid were generated using name-based UUID v3. So all instances named i1 in all clusters had the same UUID.

  With this patch UUIDs become random v4. Particular UUID values are generated in two locations:

  1. start_boot (src/lib.rs)
  2. handle_join_request_and_wait -> build_instance (src/rpc/join.rs)

  Function Instance::new() is removed in favor of explicit struct construction.

• test: refactor struct Instance inner tests

  The main purpose is to reduce the usage of Instance::new() function. It's going to be removed soon. No behavior changes are introduced yet.

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• Close #390 (closed)
• Cherry-pick to: none
• Docs follow-up: not necessary