Feedback: Made explicit non-sparse-calls count against exceededCutoff, making it easier to see when something bad happens (exceededCutoff without the explicit non-sparseness is bad)
Feature: Added DocValue support for caching of terms with low-cardinality fields (issue # 21). Not enabled as default; set facet.sparse.termlookup.maxcache to > #unique terms in the field to enable.
Added TODO for extending term cache to distributed faceting