Add new option `min_doc_count` to terms and histogram aggregations.

`min_doc_count` is the minimum number of hits that a term or histogram key
should match in order to appear in the response.

`min_doc_count=0` replaces `compute_empty_buckets` for histograms and will
behave exactly like facets' `all_terms=true` for terms aggregations.

Close #4662