[SPARK-4183] Enable NettyBlockTransferService by default

Note that we're turning this on for at least the first part of the QA period as a trial. We want to enable this (and deprecate the NioBlockTransferService) as soon as possible in the hopes that NettyBlockTransferService will be more stable and easier to maintain. We will turn it off if we run into major issues.

Author: Aaron Davidson <aaron@databricks.com>

Closes #3049 from aarondav/enable-netty and squashes the following commits:

bb981cc [Aaron Davidson] [SPARK-4183] Enable NettyBlockTransferService by default