introduced support for "shard_size" for terms & terms_stats facets. The "shard_size" is the number of term entries each shard will send back to the coordinating node. "shard_size" > "size" will increase the accuracy (both in terms of the counts associated with each term and the terms that will actually be returned the user) - of course, the higher "shard_size" is, the more expensive the processing becomes as bigger queues are maintained on a shard level and larger lists are streamed back from the shards.

closes #3821
Highlighting can return excerpt with no highlights

You can configure the highlighting api to return an excerpt of a field
even if there wasn't a match on the field.

The FVH makes excerpts from the beginning of the string to the first
boundary character after the requested length or the boundary_max_scan,
whichever comes first.  The Plain highlighter makes excerpts from the
beginning of the string to the end of the last token before the requested
length.

Closes #1171