Threadpool merge scheduler

[albertzaharovits]

This adds a new merge scheduler implementation that uses a (new) dedicated thread pool to run the merges. This way the number of concurrent merges is limited to the number of threads in the pool (i.e. the number of allocated processors to the ES JVM).

It implements dynamic IO throttling (the same target IO rate for all merges, roughly, with caveats) that's adjusted based on the number of currently active (queued + running) merges.
Smaller merges are always preferred to larger ones, irrespective of the index shard that they're coming from.
The implementation also supports the per-shard "max thread count" and "max merge count" settings, the later being used today for indexing throttling.
Note that IO throttling, max merge count, and max thread count work similarly, but not identical, to their siblings in the ConcurrentMergeScheduler.

The per-shard merge statistics are not affected, and the thread-pool statistics should reflect the merge ones (i.e. the completed thread pool stats reflects the total number of merges, across shards, per node).