[CALCITE-3111] Add RelBuilder.correlate method, and allow custom impl…

…ementations of Correlate in RelDecorrelator (Juhwan Kim)

In RelDecorrelator, refactor all Logical rels into corresponding
abstract rels, and use given RelBuilder when creating a new rel.
Add abstract rel visitors in CorelMapBuilder, and change access
levels to allow extending RelDecorrelator.

Close #1334

core/src/main/java/org/apache/calcite/plan/RelOptUtil.java

@@ -1019,6 +1019,18 @@ public static RexNode splitCorrelatedFilterCondition(
       List<RexNode> joinKeys,
       List<RexNode> correlatedJoinKeys,
       boolean extractCorrelatedFieldAccess) {
+    return splitCorrelatedFilterCondition(
+        (Filter) filter,
+        joinKeys,
+        correlatedJoinKeys,
+        extractCorrelatedFieldAccess);
+  }
+
+  public static RexNode splitCorrelatedFilterCondition(
+      Filter filter,
+      List<RexNode> joinKeys,
+      List<RexNode> correlatedJoinKeys,
+      boolean extractCorrelatedFieldAccess) {
     final List<RexNode> nonEquiList = new ArrayList<>();
 
     splitCorrelatedFilterCondition(
@@ -1371,6 +1383,22 @@ private static void splitCorrelatedFilterCondition(
       List<RexNode> correlatedJoinKeys,
       List<RexNode> nonEquiList,
       boolean extractCorrelatedFieldAccess) {
+    splitCorrelatedFilterCondition(
+        (Filter) filter,
+        condition,
+        joinKeys,
+        correlatedJoinKeys,
+        nonEquiList,
+        extractCorrelatedFieldAccess);
+  }
+
+  private static void splitCorrelatedFilterCondition(
+      Filter filter,
+      RexNode condition,
+      List<RexNode> joinKeys,
+      List<RexNode> correlatedJoinKeys,
+      List<RexNode> nonEquiList,
+      boolean extractCorrelatedFieldAccess) {
     if (condition instanceof RexCall) {
       RexCall call = (RexCall) condition;
       if (call.getOperator().getKind() == SqlKind.AND) {