A multi-tenant ConcurrentMergeScheduler

### Description

`ConcurrentMergeScheduler` computes max thread counts assuming a single `IndexWriter` in the JVM. But it's common with Solr or Elasticsearch to have tens of active `IndexWriter`s running in the same JVM. Could we make `ConcurrentMergeScheduler` more multi-tenant, or introduce a new multi-tenant merge scheduler that accounts for the fact that the JVM is shared across multiple indexes?

Start by reading ConcurrentMergeScheduler and its assumptions about max thread counts, then examine how multiple IndexWriters coexist in JVMs such as Solr or Elasticsearch. Done means an agreed multi-tenant design or scheduler approach that accounts for shared JVM resources; the issue does not name specific files or tests.

Written by the indexing model from the issue text.