Group sort by numFound [LUCENE-6126]

Currently, the doing a group query gives the response in the following manner:
grouped":{
"field_name: value":{
"matches":5,
"doclist":{"numFound":3,"start":0,"docs":[{ ... }]
}}

There should be a way to sort the groups. One way of doing it would be to store a field in the index and apply sort on that.

But, having a way to sort groups according to "numFound" would be better than storing temp fields for this purpose.

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Migrated from [LUCENE-6126](https://issues.apache.org/jira/browse/LUCENE-6126) by Prateek Sachan

The issue names no source files, tests, or entry points. Start by locating the grouping and response-sorting implementation, then determine how group ordering is represented and tested. Done means grouped results can be ordered by numFound without requiring a temporary indexed field.

Written by the indexing model from the issue text.