cfg: rename replicaset_id -> replicaset_name

Summary

Global refactoring across all picodata:

• rename replicaset_id -> replicaset_name

Environment variable change:

• PICODATA_REPLICASET_ID -> PICODATA_REPLICASET_NAME

Config file changes:

Before:

instance:
    data_dir: {data_dir}
    listen: {listen}
    ......
    replicaset_id: my-replicaset

Now:

instance:
    data_dir: {data_dir}
    listen: {listen}
    ......
    replicaset_name: my-replicaset

• Cherry-pick to: none

• Docs follow-up: docs#379 (closed)

• Follow-up for !1332 (merged)