SPARK-3470 [CORE] [STREAMING] Add Closeable / close() to Java context objects

...  that expose a stop() lifecycle method. This doesn't add `AutoCloseable`, which is Java 7+ only. But it should be possible to use try-with-resources on a `Closeable` in Java 7, as long as the `close()` does not throw a checked exception, and these don't. Q.E.D.

Author: Sean Owen <sowen@cloudera.com>

Closes #2346 from srowen/SPARK-3470 and squashes the following commits:

612c21d [Sean Owen] Add Closeable / close() to Java context objects that expose a stop() lifecycle method
Proper indent for the previous commit.