[SPARK-4307] Initialize FileDescriptor lazily in FileRegion.

Netty's DefaultFileRegion requires a FileDescriptor in its constructor, which means we need to have a opened file handle. In super large workloads, this could lead to too many open files due to the way these file descriptors are cleaned. This pull request creates a new LazyFileRegion that initializes the FileDescriptor when we are sending data for the first time.

Author: Reynold Xin <rxin@databricks.com>
Author: Reynold Xin <rxin@apache.org>

Closes #3172 from rxin/lazyFD and squashes the following commits:

0bdcdc6 [Reynold Xin] Added reference to Netty's DefaultFileRegion
d4564ae [Reynold Xin] Added SparkConf to the ctor argument of IndexShuffleBlockManager.
6ed369e [Reynold Xin] Code review feedback.
04cddc8 [Reynold Xin] [SPARK-4307] Initialize FileDescriptor lazily in FileRegion.