[SPARK-4613][Core] Java API for JdbcRDD

This PR introduces a set of Java APIs for using `JdbcRDD`:

1. Trait (interface) `JdbcRDD.ConnectionFactory`: equivalent to the `getConnection: () => Connection` parameter in `JdbcRDD` constructor.
2. Two overloaded versions of `Jdbc.create`: used to create `JavaRDD` that wraps a `JdbcRDD`.

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Author: Cheng Lian <lian@databricks.com>

Closes #3478 from liancheng/japi-jdbc-rdd and squashes the following commits:

9a54625 [Cheng Lian] Only shutdowns a single DB rather than the whole Derby driver
d4cedc5 [Cheng Lian] Moves Java JdbcRDD test case to a separate test suite
ffcdf2e [Cheng Lian] Java API for JdbcRDD

(cherry picked from commit 120a350240f58196eafcb038ca3a353636d89239)
Signed-off-by: Matei Zaharia <matei@databricks.com>