Core: let Lucene kick off merges normally

With the last Lucene 5.0.0 snapshot upgrade, I turned off CMS's hard
stalling of incoming threads when merging can't keep up, since ES does
its own index throttling to deal with this.

This means we can simplify a lot of ES's custom wrapper code for
handling segment merges: remove EnableMergeScheduler, MERGE thread
pool, InternalIndexShard.EngineMerger, and the settings
index.merge.force_async_merge and index.merge.async_interval.

Ie, we should now just let Lucene kick off merges as soon as they are
needed, instead of polling every 1.0 second and doing it ourselves.