Core: let Lucene kick off merges

Today, Elasticsearch has a separate merge thread pool checking once
per second (by default) if any merges are necessary, but this is no
longer necessary since we can and do now tell Lucene's
ConcurrentMergeScheduler never to "hard pause" threads when merges
fall behind, since we do our own index throttling.

This change goes back to letting Lucene launch merges as needed, and
removes these two expert settings:

  index.merge.force_async_merge
  index.merge.async_interval

Now merges kick off immediately instead of waiting up to 1 second
before running.

Closes #8643