[SPARK-2805] Upgrade to akka 2.3.4

Upgrade to akka 2.3.4

Author: Anand Avati <avati@redhat.com>

Closes #1685 from avati/SPARK-1812-akka-2.3 and squashes the following commits:

57a2315 [Anand Avati] SPARK-1812: streaming - remove tests which depend on akka.actor.IO
2a551d3 [Anand Avati] SPARK-1812: core - upgrade to akka 2.3.4
[SPARK-3844][UI] Truncate appName in WebUI if it is too long

Truncate appName in WebUI if it is too long.

Author: Xiangrui Meng <meng@databricks.com>

Closes #2707 from mengxr/truncate-app-name and squashes the following commits:

87834ce [Xiangrui Meng] move scala import below java
c7111dc [Xiangrui Meng] truncate appName in WebUI if it is too long
add spark.driver.memory to config docs

It took me a minute to track this down, so I thought it could be useful to have it in the docs.

I'm unsure if 512mb is the default for spark.driver.memory? Also - there could be a better value for the 'description' to differentiate it from spark.executor.memory.

Author: nartz <nartzpod@gmail.com>
Author: Nathan Artz <nathanartz@Nathans-MacBook-Pro.local>

Closes #2410 from nartz/docs/add-spark-driver-memory-to-config-docs and squashes the following commits:

a2f6c62 [nartz] Update configuration.md
74521b8 [Nathan Artz] add spark.driver.memory to config docs