SPARK-3612. Executor shouldn't quit if heartbeat message fails to reach ...

...the driver

Author: Sandy Ryza <sandy@cloudera.com>

Closes #2487 from sryza/sandy-spark-3612 and squashes the following commits:

2b7353d [Sandy Ryza] SPARK-3612. Executor shouldn't quit if heartbeat message fails to reach the driver
(cherry picked from commit d79238d03a2ffe0cf5fc6166543d67768693ddbe)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>