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.