Make indexRandom handle many documents better

* Index one at a time only rarely if doing more then 300.
* When launching async actions, take some care to make sure you don't already
have more then 150 other async actions in flight.
* When indexing in bulk split into chunks of 1000 documents.
Speed up phrase suggestion scoring

Two changes:
1.  In the StupidBackoffScorer only look for the trigram if there is a bigram.
2.  Cache the frequencies in WordScorer so we don't look them up again and
again and again.  This is implemented by wrapping the TermsEnum in a special
purpose wrapper that really only works in context of the WordScorer.

This provides a pretty substantial speedup when there are many candidates.

Closes #5395
Move to use serial merge schedule by default
Today, we use ConcurrentMergeScheduler, and this can be painful since it is concurrent on a shard level, with a max of 3 threads doing concurrent merges. If there are several shards being indexed, then there will be a minor explosion of threads trying to do merges, all being throttled by our merge throttling.
Moving to serial merge scheduler will still maintain concurrency of merges across shards, as we have the merge thread pool that schedules those merges. It will just be a serial one on a specific shard.
Also, on serial merge scheduler, we now have a limit of how many merges it will do at one go, so it will let other shards get their fair chance of merging. We use the pending merges on IW to check if merges are needed or not for it.
Note, that if a merge is happening, it will not block due to a sync on the maybeMerge call at indexing (flush) time, since we wrap our merge scheduler with the EnabledMergeScheduler, where maybeMerge is not activated during indexing, only with explicit calls to IW#maybeMerge (see Merges).
closes #5447
Geo Point Fieldmapper: Allow distance for geohash precision

Even though mentioned differently in the docs, the geohash precision needed to
be an integer instead of a DistanceUnit.

Closes #5448
[TEST] Added the option to specify on what ES version a node should run with. Useful for testing.