parent/child: Move over to Lucene's join util and doc values by default

This a breaking change:
1) A parent type needs be marked as parent in the _parent field mapping of the parent type.
2) top_children query will be removed. The top_children query was somewhat an alternative to has_child when it came to speed, but it isn't accurate and wasn't always faster.

Indices created before 2.0 will use field data and the old way of executing queries, but indices created on or after 2.0 will use the Lucene join.

Closes elastic#6107
Closes elastic#6511
Closes elastic#8134

src/main/java/org/apache/lucene/join/GlobalOrdinalsCollector.java

@@ -0,0 +1,116 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.lucene.join;
+
+import org.apache.lucene.index.DocValues;
+import org.apache.lucene.index.LeafReaderContext;
+import org.apache.lucene.index.MultiDocValues;
+import org.apache.lucene.index.SortedDocValues;
+import org.apache.lucene.search.Collector;
+import org.apache.lucene.search.LeafCollector;
+import org.apache.lucene.search.Scorer;
+import org.apache.lucene.util.LongBitSet;
+import org.apache.lucene.util.LongValues;
+
+import java.io.IOException;
+
+/**
+ * A collector that collects all ordinals from a specified field matching the query.
+ *
+ * @lucene.experimental
+ */
+final class GlobalOrdinalsCollector implements Collector {
+
+  final String field;
+  final LongBitSet collectedOrds;
+  final MultiDocValues.OrdinalMap ordinalMap;
+
+  GlobalOrdinalsCollector(String field, MultiDocValues.OrdinalMap ordinalMap, long valueCount) {
+    this.field = field;
+    this.ordinalMap = ordinalMap;
+    this.collectedOrds = new LongBitSet(valueCount);
+  }
+
+  public LongBitSet getCollectorOrdinals() {
+    return collectedOrds;
+  }
+
+  @Override
+  public boolean needsScores() {
+    return false;
+  }
+
+  @Override
+  public LeafCollector getLeafCollector(LeafReaderContext context) throws IOException {
+    SortedDocValues docTermOrds = DocValues.getSorted(context.reader(), field);
+    if (ordinalMap != null) {
+      LongValues segmentOrdToGlobalOrdLookup = ordinalMap.getGlobalOrds(context.ord);
+      return new OrdinalMapCollector(docTermOrds, segmentOrdToGlobalOrdLookup);
+    } else {
+      return new SegmentOrdinalCollector(docTermOrds);
+    }
+  }
+
+  final class OrdinalMapCollector implements LeafCollector {
+
+    private final SortedDocValues docTermOrds;
+    private final LongValues segmentOrdToGlobalOrdLookup;
+
+    OrdinalMapCollector(SortedDocValues docTermOrds, LongValues segmentOrdToGlobalOrdLookup) {
+      this.docTermOrds = docTermOrds;
+      this.segmentOrdToGlobalOrdLookup = segmentOrdToGlobalOrdLookup;
+    }
+
+    @Override
+    public void collect(int doc) throws IOException {
+      final long segmentOrd = docTermOrds.getOrd(doc);
+      if (segmentOrd != -1) {
+        final long globalOrd = segmentOrdToGlobalOrdLookup.get(segmentOrd);
+        collectedOrds.set(globalOrd);
+      }
+    }
+
+    @Override
+    public void setScorer(Scorer scorer) throws IOException {
+    }
+  }
+
+  final class SegmentOrdinalCollector implements LeafCollector {
+
+    private final SortedDocValues docTermOrds;
+
+    SegmentOrdinalCollector(SortedDocValues docTermOrds) {
+      this.docTermOrds = docTermOrds;
+    }
+
+    @Override
+    public void collect(int doc) throws IOException {
+      final long segmentOrd = docTermOrds.getOrd(doc);
+      if (segmentOrd != -1) {
+        collectedOrds.set(segmentOrd);
+      }
+    }
+
+    @Override
+    public void setScorer(Scorer scorer) throws IOException {
+    }
+  }
+
+}