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.
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
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
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
During parent uid loading seek to next parent type when child type is encountered.
Relates to #3028