[SPARK-4847][SQL]Fix "extraStrategies cannot take effect in SQLContext" issue

Author: jerryshao <saisai.shao@intel.com>

Closes #3698 from jerryshao/SPARK-4847 and squashes the following commits:

4741130 [jerryshao] Make later added extraStrategies effect when calling strategies

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

@@ -327,7 +327,7 @@ class SQLContext(@transient val sparkContext: SparkContext)
 
     def numPartitions = self.numShufflePartitions
 
-    val strategies: Seq[Strategy] =
+    def strategies: Seq[Strategy] =
       extraStrategies ++ (
       CommandStrategy(self) ::
       DataSourceStrategy ::

sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveContext.scala

@@ -338,7 +338,7 @@ class HiveContext(sc: SparkContext) extends SQLContext(sc) {
   val hivePlanner = new SparkPlanner with HiveStrategies {
     val hiveContext = self
 
-    override val strategies: Seq[Strategy] = extraStrategies ++ Seq(
+    override def strategies: Seq[Strategy] = extraStrategies ++ Seq(
       DataSourceStrategy,
       CommandStrategy(self),
       HiveCommandStrategy(self),