Fielddata: goodbye comparators.

This commit removes custom comparators in favor of the ones that are in Lucene.

The major change is for nested documents: instead of having a comparator wrapper
that deals with nested documents, this is done at the fielddata level by having
a selector that returns the value to use for comparison.

Sorting with custom missing string values might be slower since it is using
TermValComparator since Lucene's TermOrdValComparator only supports sorting
missing values first or last. But other than this particular case, this change
will allow us to benefit from improvements on comparators from the Lucene side.

Close #5980
[TEST] Don't recycle in facets.

The recycling happening in facets is done manually and arrays are sometimes not
released. Aggregations do it in a less error-prone way by registering on to the
SearchContext.
Change numeric data types to use SORTED_NUMERIC docvalues type
instead of a custom encoding in BINARY.

In low level benchmarks this is 2x to 5x faster: its also optimized
for the common case where fields actually only contain at most one
value for each document.

Additionally SORTED_NUMERIC doesn't lose values if they appear more
than once, so mathematical computations such as averages are correct.

Closes #6967
Search & Count: Add option to early terminate doc collection

Allow users to control document collection termination, if a specified terminate_after number is
set. Upon setting the newly added parameter, the response will include a boolean terminated_early
flag, indicating if the document collection for any shard terminated early.

closes #6876