Clean up wording to reduce confusion

The description of the timeout parameter was worded misleadingly; it implied that the API would wait until the cluster reached the desired level and then stayed at that level for the timeout. I've tweaked the sentence to remove the risk of confusion.
Merge pull request #3940 from rboulton/patch-1

[Docs] Clean up wording in cluster health api doc
Java API: Setting track scores does not affect scan search type
When setting track scores, the scan search type will return the scores for each document. The Java API builder does not properly set this value (it only sets it if a sort in in place, which is not relevant for scan search type).

closes #3949
Add doc values to TermsFacetSearchBenchmark.
Refactor FieldData iterations

This commit primarily folds [Double|Bytes|Long|GeoPoint]Values.Iter
into [Double|Bytes|Long|GeoPoint]Values. Iterations now don't require
a auxillary class (Iter) but instead driven by native for loops. All
[Double|Bytes|Long|GeoPoint]Values are stateful and provide `setDocId`
and `nextValue` methods to iterate over all values in a document.
This has several advantage:

 * The amout of specialized classes is reduced
 * Iteration is clearly stateful ie. Iters can't be confused to be local.
 * All iterations are size bounded which prevents runtime checks and
   allows JIT optimizations / loop un-rolling and most iterations are
   branch free.
 * Due to the bounded iteration the need for a `hasNext` method call
   is removed.
 * Value iterations feels more native.

This commit also adds consistent documentation and unifies the calcualtion
if SortMode is involved.

This commit also changes the runtime behavior of BytesValues#getValue() such that it
will never return `null` anymore. If a document has no value in a field
this method still returns a `BytesRef` with a `length` of 0. To identify
documents with no values #hasValue() or #setDocument(int) should be used.
The latter should be preferred if the value will be consumed in the case
the document has a value.