[SPARK-8463][SQL] Use DriverRegistry to load jdbc driver at writing path

JIRA: https://issues.apache.org/jira/browse/SPARK-8463

Currently, at the reading path, `DriverRegistry` is used to load needed jdbc driver at executors. However, at the writing path, we also need `DriverRegistry` to load jdbc driver.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #6900 from viirya/jdbc_write_driver and squashes the following commits:

16cd04b [Liang-Chi Hsieh] Use DriverRegistry to load jdbc driver at writing path.

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

@@ -58,13 +58,12 @@ package object jdbc {
      * are used.
      */
     def savePartition(
-        url: String,
+        getConnection: () => Connection,
         table: String,
         iterator: Iterator[Row],
         rddSchema: StructType,
-        nullTypes: Array[Int],
-        properties: Properties): Iterator[Byte] = {
-      val conn = DriverManager.getConnection(url, properties)
+        nullTypes: Array[Int]): Iterator[Byte] = {
+      val conn = getConnection()
       var committed = false
       try {
         conn.setAutoCommit(false) // Everything in the same db transaction.
@@ -185,8 +184,10 @@ package object jdbc {
       }
 
       val rddSchema = df.schema
+      val driver: String = DriverRegistry.getDriverClassName(url)
+      val getConnection: () => Connection = JDBCRDD.getConnector(driver, url, properties)
       df.foreachPartition { iterator =>
-        JDBCWriteDetails.savePartition(url, table, iterator, rddSchema, nullTypes, properties)
+        JDBCWriteDetails.savePartition(getConnection, table, iterator, rddSchema, nullTypes)
       }
     }