[SPARK-2350] Don't NPE while launching drivers

Prior to this change, we could throw a NPE if we launch a driver while another one is waiting, because removing from an iterator while iterating over it is not safe.

Author: Aaron Davidson <aaron@databricks.com>

Closes #1289 from aarondav/master-fail and squashes the following commits:

1cf1cf4 [Aaron Davidson] SPARK-2350: Don't NPE while launching drivers
(cherry picked from commit 586feb5c9528042420f678f78bacb6c254a5eaf8)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>
SPARK-2282: Reuse PySpark Accumulator sockets to avoid crashing Spark

JIRA: https://issues.apache.org/jira/browse/SPARK-2282

This issue is caused by a buildup of sockets in the TIME_WAIT stage of TCP, which is a stage that lasts for some period of time after the communication closes.

This solution simply allows us to reuse sockets that are in TIME_WAIT, to avoid issues with the buildup of the rapid creation of these sockets.

Author: Aaron Davidson <aaron@databricks.com>

Closes #1220 from aarondav/SPARK-2282 and squashes the following commits:

2e5cab3 [Aaron Davidson] SPARK-2282: Reuse PySpark Accumulator sockets to avoid crashing Spark
(cherry picked from commit 97a0bfe1c0261384f09d53f9350de52fb6446d59)

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