[SQL][minor] rename apply for QueryPlanner

A follow-up of https://github.com/apache/spark/pull/5624

Author: Wenchen Fan <cloud0fan@outlook.com>

Closes #6142 from cloud-fan/tmp and squashes the following commits:

971a92b [Wenchen Fan] use plan instead of execute
24c5ffe [Wenchen Fan] rename apply

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/planning/QueryPlanner.scala

@@ -51,9 +51,9 @@ abstract class QueryPlanner[PhysicalPlan <: TreeNode[PhysicalPlan]] {
    * filled in automatically by the QueryPlanner using the other execution strategies that are
    * available.
    */
-  protected def planLater(plan: LogicalPlan) = apply(plan).next()
+  protected def planLater(plan: LogicalPlan) = this.plan(plan).next()
 
-  def apply(plan: LogicalPlan): Iterator[PhysicalPlan] = {
+  def plan(plan: LogicalPlan): Iterator[PhysicalPlan] = {
     // Obviously a lot to do here still...
     val iter = strategies.view.flatMap(_(plan)).toIterator
     assert(iter.hasNext, s"No plan for $plan")

sql/core/src/main/scala/org/apache/spark/sql/SQLContext.scala

@@ -1321,7 +1321,7 @@ class SQLContext(@transient val sparkContext: SparkContext)
     // TODO: Don't just pick the first one...
     lazy val sparkPlan: SparkPlan = {
       SparkPlan.currentContext.set(self)
-      planner(optimizedPlan).next()
+      planner.plan(optimizedPlan).next()
     }
     // executedPlan should not be used to initialize any SparkPlan. It should be
     // only used for execution.