Scripting: Add script engine for lucene expressions

Adds support for new "expression" script engine.

This is backed by lucene expressions.  These are javascript expressions, which can only access numeric fielddata, parameters, and _score.  They can only be used for searches (not document updates).

closes #6818
I think it is cleaner to keep this separate for now, until the real setScorer can be fixed here to pass a real/canned scorer?