EarlyTerminatingSortingCollector support for grouped searches [LUCENE-7341]

Currently grouped searches must not use the early terminating sorting collector because the wrong results would be returned. This ticket proposes to change the `EarlyTerminatingSortingCollector` class and probably the `LeafCollector` interface to support early termination for grouped searches.

**Illustration (aaa is an as yet unnamed boolean flag):**

```Java
# fictitious (sorted by X) segment
| doc key | D0 D1 D2 D3 D4 D5 ... D10 D11 D12 D13 D14 D15 ... D20 D21 D22 D23 D24 D25 ...
| doc grp | G0 G0 G0 G0 G0 G0 ... D10 G10 G10 G10 G10 G10 ... G20 G20 G20 G20 G20 G20 ...
```

```Java
# query with rows=3 sort=X group=false
| query result | D0 D1 D2

# existing code:
# use a EarlyTerminatingSortingCollector with numDocsToCollect=3
# EarlyTerminatingSortingCollector.getLeafCollector returns a LeafCollector
# whose collect method uses (++numCollected >= numDocsToCollect) as the terminating condition
```

```Java
# query with rows=3 sort=X group=true group.field=grp group.sort=X group.limit=1
| query result | G0(D0) G10(D10) G20(D20)

# existing code:
# cannot use EarlyTerminatingSortingCollector (query result would wrongly be just 'G0(D0)')
# proposed new code:
# use a EarlyTerminatingSortingCollector(... numDocsToCollect=3 aaa=true)
```

```Java
# query with rows=3 sort=X group=true group.field=grp group.sort=X group.limit=5
| query result | G0(D0,D1,D2,D3,D4) G10(D10,D11,D12,D13,D14) G20(D20,D21,D22,D23,D24)

# existing code:
# cannot use EarlyTerminatingSortingCollector (query result would wrongly be just 'G0(D0,D1,D2)')
# proposed new code:
# use a EarlyTerminatingSortingCollector(... numDocsToCollect=3 aaa=true)
```

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Migrated from [LUCENE-7341](https://issues.apache.org/jira/browse/LUCENE-7341) by Christine Poerschke (@cpoerschke), 1 vote, updated Jun 17 2016