[SPARK-11563][CORE][REPL] Use RpcEnv to transfer REPL-generated classes.

This avoids bringing up yet another HTTP server on the driver, and
instead reuses the file server already managed by the driver's
RpcEnv. As a bonus, the repl now inherits the security features of
the network library.

There's also a small change to create the directory for storing classes
under the root temp dir for the application (instead of directly
under java.io.tmpdir).

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #9923 from vanzin/SPARK-11563.