[SPARK-26598][SQL] Fix HiveThriftServer2 cannot be modified hiveconf/hivevar variables

### What changes were proposed in this pull request?
The intent to use the --hiveconf/--hivevar parameter is just an initialization value, so setting it once in ```SparkSQLSessionManager#openSession``` is sufficient, and each time the ```SparkExecuteStatementOperation``` setting causes the variable to not be modified.

### Why are the changes needed?
It is wrong to set the --hivevar/--hiveconf variable in every ```SparkExecuteStatementOperation```, which prevents variable updates.

### Does this PR introduce any user-facing change?
```
cat <<EOF > test.sql
select '\${a}', '\${b}';
set b=bvalue_MOD_VALUE;
set b;
EOF

beeline -u jdbc:hive2://localhost:10000 --hiveconf a=avalue --hivevar b=bvalue -f test.sql
```
current result:
```
+-----------------+-----------------+--+
|     avalue      |     bvalue      |
+-----------------+-----------------+--+
| avalue          | bvalue          |
+-----------------+-----------------+--+
+-----------------+-----------------+--+
|       key       |      value      |
+-----------------+-----------------+--+
| b               | bvalue          |
+-----------------+-----------------+--+
1 row selected (0.022 seconds)
```
after modification:
```
+-----------------+-----------------+--+
|     avalue      |     bvalue      |
+-----------------+-----------------+--+
| avalue          | bvalue          |
+-----------------+-----------------+--+
+-----------------+-----------------+--+
|       key       |      value      |
+-----------------+-----------------+--+
| b               | bvalue_MOD_VALUE|
+-----------------+-----------------+--+
1 row selected (0.022 seconds)
```

### How was this patch tested?
modified the existing unit test

Closes #25722 from cxzl25/fix_SPARK-26598.

Authored-by: sychen <sychen@ctrip.com>
Signed-off-by: Yuming Wang <wgyumg@gmail.com>

Author: cxzl25

sql/hive-thriftserver/src/main/scala/org/apache/spark/sql/hive/thriftserver/SparkSQLSessionManager.scala

@@ -63,6 +63,9 @@ private[hive] class SparkSQLSessionManager(hiveServer: HiveServer2, sqlContext:
       sqlContext.newSession()
     }
     ctx.setConf(HiveUtils.FAKE_HIVE_VERSION.key, HiveUtils.builtinHiveVersion)
+    val hiveSessionState = session.getSessionState
+    setConfMap(ctx, hiveSessionState.getOverriddenConfigurations)
+    setConfMap(ctx, hiveSessionState.getHiveVariables)
     if (sessionConf != null && sessionConf.containsKey("use:database")) {
       ctx.sql(s"use ${sessionConf.get("use:database")}")
     }
@@ -76,4 +79,12 @@ private[hive] class SparkSQLSessionManager(hiveServer: HiveServer2, sqlContext:
     sparkSqlOperationManager.sessionToActivePool.remove(sessionHandle)
     sparkSqlOperationManager.sessionToContexts.remove(sessionHandle)
   }
+
+  def setConfMap(conf: SQLContext, confMap: java.util.Map[String, String]): Unit = {
+    val iterator = confMap.entrySet().iterator()
+    while (iterator.hasNext) {
+      val kv = iterator.next()
+      conf.setConf(kv.getKey, kv.getValue)
+    }
+  }
 }

sql/hive-thriftserver/src/main/scala/org/apache/spark/sql/hive/thriftserver/server/SparkSQLOperationManager.scala

@@ -28,7 +28,6 @@ import org.apache.spark.internal.Logging
 import org.apache.spark.sql.SQLContext
 import org.apache.spark.sql.hive.HiveUtils
 import org.apache.spark.sql.hive.thriftserver._
-import org.apache.spark.sql.internal.SQLConf
 
 /**
  * Executes queries using Spark SQL, and maintains a list of handles to active queries.
@@ -51,9 +50,6 @@ private[thriftserver] class SparkSQLOperationManager()
     require(sqlContext != null, s"Session handle: ${parentSession.getSessionHandle} has not been" +
       s" initialized or had already closed.")
     val conf = sqlContext.sessionState.conf
-    val hiveSessionState = parentSession.getSessionState
-    setConfMap(conf, hiveSessionState.getOverriddenConfigurations)
-    setConfMap(conf, hiveSessionState.getHiveVariables)
     val runInBackground = async && conf.getConf(HiveUtils.HIVE_THRIFT_SERVER_ASYNC)
     val operation = new SparkExecuteStatementOperation(parentSession, statement, confOverlay,
       runInBackground)(sqlContext, sessionToActivePool)
@@ -144,12 +140,4 @@ private[thriftserver] class SparkSQLOperationManager()
     logDebug(s"Created GetFunctionsOperation with session=$parentSession.")
     operation
   }
-
-  def setConfMap(conf: SQLConf, confMap: java.util.Map[String, String]): Unit = {
-    val iterator = confMap.entrySet().iterator()
-    while (iterator.hasNext) {
-      val kv = iterator.next()
-      conf.setConfString(kv.getKey, kv.getValue)
-    }
-  }
 }

sql/hive-thriftserver/src/test/scala/org/apache/spark/sql/hive/thriftserver/HiveThriftServer2Suites.scala

@@ -144,10 +144,17 @@ class HiveThriftBinaryServerSuite extends HiveThriftJdbcTest {
       def executeTest(hiveList: String): Unit = {
         hiveList.split(";").foreach{ m =>
           val kv = m.split("=")
-          // select "${a}"; ---> avalue
-          val resultSet = statement.executeQuery("select \"${" + kv(0) + "}\"")
+          val k = kv(0)
+          val v = kv(1)
+          val modValue = s"${v}_MOD_VALUE"
+          // select '${a}'; ---> avalue
+          val resultSet = statement.executeQuery(s"select '$${$k}'")
           resultSet.next()
-          assert(resultSet.getString(1) === kv(1))
+          assert(resultSet.getString(1) === v)
+          statement.executeQuery(s"set $k=$modValue")
+          val modResultSet = statement.executeQuery(s"select '$${$k}'")
+          modResultSet.next()
+          assert(modResultSet.getString(1) === s"$modValue")
         }
       }
     }