This adds a new boolean (index.merge.scheduler.auto_throttle) dynamic
setting, default true (matching Lucene), to adaptively set the IO rate
limit for merges over time.
This is more flexible than the previous fixed rate throttling because
it responds depending on the incoming merge rate, so search-heavy
applications that are not doing much indexing will see merges heavily
throttled while indexing-heavy cases will lighten the throttle so
merges can keep up within incoming indexing.
The fixed rate throttling is still available as a fallback if things
go horribly wrong.
Closes#9243Closes#9133