SOLR-17076: Optimize placement factory get all replica

[patsonluk]

https://issues.apache.org/jira/browse/SOLR-17076

Description

OrderedNodePlacementPlugin#getAllReplicasOnNode can be slow in a cluster with lot of replicas. The effect could compound with new collection creation with many shards.

Solution

Introduced a new field allReplicas which keeps track of all the replicas added/removed in the Plugin. Return a copy of that set instead for getAllReplicasOnNode.

Also added a new convenient method getAllReplicaCount if only the count is needed

Tests

A minor change, rely on existing unit test case

Checklist

Please review the following and check all that apply:

• I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
• I have created a Jira issue and added the issue ID to my pull request title.
• I have given Solr maintainers access to contribute to my PR branch. (optional but recommended)
• I have developed this patch against the main branch.
• I have run ./gradlew check.
• I have added tests for my changes.
• I have added documentation for the Reference Guide

[patsonluk]

Somehow opened against our main instead of apache's....closing this!