Fixed `java.sql.SQLException: No suitable driver found` when loading DataFrame into Spark SQL if the driver is supplied with `--jars` argument.

The problem is in `java.sql.DriverManager` class that can't access drivers loaded by Spark ClassLoader.

Wrappers that forward requests are created for these drivers.

Also, it's not necessary any more to include JDBC drivers in `--driver-class-path` in local mode, specifying in `--jars` argument is sufficient.

Author: Vyacheslav Baranov <slavik.baranov@gmail.com>

Closes #5782 from SlavikBaranov/SPARK-6913 and squashes the following commits:

510c43f [Vyacheslav Baranov] [SPARK-6913] Fixed review comments
b2a727c [Vyacheslav Baranov] [SPARK-6913] Fixed thread race on driver registration
c8294ae [Vyacheslav Baranov] [SPARK-6913] Fixed "No suitable driver found" when using using JDBC driver added with SparkContext.addJar