[SPARK-17955][SQL] Make DataFrameReader.jdbc call DataFrameReader.format("jdbc").load

## What changes were proposed in this pull request?

This PR proposes to make `DataFrameReader.jdbc` call `DataFrameReader.format("jdbc").load` consistently with other APIs in `DataFrameReader`/`DataFrameWriter` and avoid calling `sparkSession.baseRelationToDataFrame(..)` here and there.

The changes were mostly copied from `DataFrameWriter.jdbc()` which was recently updated.

```diff
-    val params = extraOptions.toMap ++ connectionProperties.asScala.toMap
-    val options = new JDBCOptions(url, table, params)
-    val relation = JDBCRelation(parts, options)(sparkSession)
-    sparkSession.baseRelationToDataFrame(relation)
+    this.extraOptions = this.extraOptions ++ connectionProperties.asScala
+    // explicit url and dbtable should override all
+    this.extraOptions += ("url" -> url, "dbtable" -> table)
+    format("jdbc").load()
```

## How was this patch tested?

Existing tests should cover this.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #15499 from HyukjinKwon/SPARK-17955.

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

@@ -232,10 +232,10 @@ class DataFrameReader private[sql](sparkSession: SparkSession) extends Logging {
       parts: Array[Partition],
       connectionProperties: Properties): DataFrame = {
     // connectionProperties should override settings in extraOptions.
-    val params = extraOptions.toMap ++ connectionProperties.asScala.toMap
-    val options = new JDBCOptions(url, table, params)
-    val relation = JDBCRelation(parts, options)(sparkSession)
-    sparkSession.baseRelationToDataFrame(relation)
+    this.extraOptions = this.extraOptions ++ connectionProperties.asScala
+    // explicit url and dbtable should override all
+    this.extraOptions += ("url" -> url, "dbtable" -> table)
+    format("jdbc").load()
   }
 
   /**

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

@@ -426,8 +426,8 @@ final class DataFrameWriter[T] private[sql](ds: Dataset[T]) {
   def jdbc(url: String, table: String, connectionProperties: Properties): Unit = {
     assertNotPartitioned("jdbc")
     assertNotBucketed("jdbc")
-    // connectionProperties should override settings in extraOptions
-    this.extraOptions = this.extraOptions ++ (connectionProperties.asScala)
+    // connectionProperties should override settings in extraOptions.
+    this.extraOptions = this.extraOptions ++ connectionProperties.asScala
     // explicit url and dbtable should override all
     this.extraOptions += ("url" -> url, "dbtable" -> table)
     format("jdbc").save()