[SQL] Set sessionState in QueryExecution.

This PR sets the SessionState in HiveContext's QueryExecution. So, we can make sure that SessionState.get can return the SessionState every time.

Author: Yin Huai <yhuai@databricks.com>

Closes #4445 from yhuai/setSessionState and squashes the following commits:

769c9f1 [Yin Huai] Remove unused import.
439f329 [Yin Huai] Try again.
427a0c9 [Yin Huai] Set SessionState everytime when we create a QueryExecution in HiveContext.
a3b7793 [Yin Huai] Set sessionState when dealing with CreateTableAsSelect.

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

@@ -424,6 +424,11 @@ class HiveContext(sc: SparkContext) extends SQLContext(sc) {
   /** Extends QueryExecution with hive specific features. */
   protected[sql] class QueryExecution(logicalPlan: LogicalPlan)
     extends super.QueryExecution(logicalPlan) {
+    // Like what we do in runHive, makes sure the session represented by the
+    // `sessionState` field is activated.
+    if (SessionState.get() != sessionState) {
+      SessionState.start(sessionState)
+    }
 
     /**
      * Returns the result as a hive compatible sequence of strings.  For native commands, the