Reduce branches in TopChildrenQuery

The branches used in the score method can be moved into the
scorer call and be essentially a constant operation rather than
a linear operation depending on the number of parent docs.
Ensure tests pass if store dir is a soft-link
Support SpanMultiTerm, closes #2610, #2400

This adds support for lucene span multi term queries. This lucene query
allows users to form complicated queries such as wildcards or prefix
queries embedded within span queries.
Refactoring SpanMultiTermQuery support

* Added license headers where needed
* Refactored SpanMultiTermQueryParser
* Refactored tests to adhere to other tests
Renaming span_multi_term query to span_multi

... due to discussing this on #2610 in order to have a more concise name
Not load the ids of child documents into memory.
Closes #3028
Expose LimitTokenCountFilter as a TokenFilter

Closes #3013
Added test for LimitTokenCountFilterFactory
Allow to disable allocation on the index level
Similar to the global cluster wide disable allocation flags, allow to set those on a specific index by updating its settings. The keys are the same as the cluster one, except they start with an index, for example: index.routing.allocation.disable_allocation set to true.
closes #3031
Make sure only relevant documents are evaluated in the second round lookup phase.

Both has_parent and has_child filters are internally executed in two rounds. In the second round all documents are evaluated whilst only specific documents need to be checked. In the has_child case only documents belonging to a specific parent type need to be checked and in the has_parent case only child documents need to be checked.

Closes #3034
Support for term facets on unmapped fields

Added support for unmapped & partially mapped fields (partially mapped fields may occur when searching across multiple indices where the faceted field is mapped on some and unmapped on others). If a shard doesn't have mappings for a field, the matching documents count on that shard will be added to the missing count for that facet.
Use Recovery Throtteling by default.

To prevent to extensive resource use during recovery we use
recovery throtteling by default to prevent unexpected peak load
on clusters. The default is set to 20 MB/sec.

Closes #3035
Use Merge Throtteling by default on node level.

Merge Throtteling is one of the most recommended settings and crucial in the
RealTime indexing case. We should set the default to a reasonable setting
that allows folks to index in a production index and don't see large merge
peaks by default. The default is set to 20 MB/sec on the node level.

Closes #3033
Raise search threadpool default size.

The default size used to be 2x availableProcessors which seemed to
be a to lowish value in practice. 3x appeared to be a sweetspot for
most application. The default is now 3 x availableProcessors

Closes #3023
During parent uid loading seek to next parent type when child type is encountered.
Relates to #3028
use bytes instead of String as key in versionMap
no need to create a String every time we put or get a value from the version map
remove unused class
reuse version key in an actual operation
no need to compute the hash several times
clean thread locals without needing a wrapper
clean thread locals smartly by identifying "our" classes, and removing them, so there is no need to wrap it in our our clenable values
Routing value not serialized on transport layer.
Closes #3039