cfg: rename cluster_id -> cluster_name, instance_id -> instance_name

Summary

Global refactoring across all picodata:

• Rename instance_id -> instance_name
• Rename cluster_id -> cluster_name
• Rename cluster.cluster_name -> cluster.name
• Rename cluster_cluster_name -> cluster_name
• Rename instance.instance_uuid -> instance.uuid
• Rename replicaset.replicaset_uuid -> replicaset.uuid

Environment variables rename:

• PICODATA_CLUSTER_ID -> PICODATA_CLUSTER_NAME
• PICODATA_INSTANCE_ID -> PICODATA_INSTANCE_NAME

Config file changes:

Before:

    cluster:
        cluster_id: test
    instance:
        instance_id: i1

Now:

    cluster:
        name: test
    instance:
        name: i1

• Close #981 (closed)
• Cherry-pick to: none
• Docs follow-up: docs#379 (closed)

Closes #981 (closed)