In LUCENE-5310 we explored improving CMS/SMS sharing/concurrency, but
the issue never "converged", so I want to break out one small part of
it here: the ability to override CMS's default "aggressive"
denial-of-service protection where it forcefully stalls the incoming
threads that are responsible for creating too many segments.
More advanced applications can more gracefully handle the "too many
merges" by e.g. slowing down the incoming indexing rate at a higher