[SPARK-2677] BasicBlockFetchIterator#next can wait forever

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #1632 from sarutak/SPARK-2677 and squashes the following commits:

cddbc7b [Kousuke Saruta] Removed Exception throwing when ConnectionManager#handleMessage receives ack for non-referenced message
d3bd2a8 [Kousuke Saruta] Modified configuration.md for spark.core.connection.ack.timeout
e85f88b [Kousuke Saruta] Removed useless synchronized blocks
7ed48be [Kousuke Saruta] Modified ConnectionManager to use ackTimeoutMonitor ConnectionManager-wide
9b620a6 [Kousuke Saruta] Merge branch 'master' of git://git.apache.org/spark into SPARK-2677
0dd9ad3 [Kousuke Saruta] Modified typo in ConnectionManagerSuite.scala
7cbb8ca [Kousuke Saruta] Modified to match with scalastyle
8a73974 [Kousuke Saruta] Merge branch 'master' of git://git.apache.org/spark into SPARK-2677
ade279a [Kousuke Saruta] Merge branch 'master' of git://git.apache.org/spark into SPARK-2677
0174d6a [Kousuke Saruta] Modified ConnectionManager.scala to handle the case remote Executor cannot ack
a454239 [Kousuke Saruta] Merge branch 'master' of git://git.apache.org/spark into SPARK-2677
9b7b7c1 [Kousuke Saruta] (WIP) Modifying ConnectionManager.scala