This patch piggy-back's on vanzin's work to simplify the Guava shading,
and adds Jetty as a shaded library in Spark. Other than adding Jetty,
it consilidates the \<artifactSet\>'s into the root pom. I found it was
a bit easier to follow that way, since you don't need to look into
child pom's to find out specific artifact sets included in shading.

Author: Patrick Wendell <patrick@databricks.com>

Closes #4252 from pwendell/jetty and squashes the following commits:

19f0710 [Patrick Wendell] More code review feedback
961452d [Patrick Wendell] Responding to feedback from Marcello
6df25ca [Patrick Wendell] [WIP] [SPARK-3996]: Shade Jetty in Spark deliverables