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.

Can you also remove it from the docs (`docs/reference/cat/thread_pool.asciidoc`)?