Added a disk identifier to a block's metadata.

For every block that it writes to disk, a BlockManager now keeps track
of a String identifier that specifies the physical disk to which that
block was written. This identifier is the name of the FileStore to which
a block was written, and is not necessarily globally unique.
Added Shivaram to default reviewers.

This commit also updates Kay's email address to use gmail rather
than EECS.
Removed Netty shuffle implementation.

This commit removes the netty shuffle implementation (the default
shuffle implementation uses Java's NIO library).

According to Patrick Wendell, this never worked well anyway (there
were a bunch of corner cases it didn't support, and in the more
recent Spark branch this code was completely rewritten:
https://github.com/apache/spark/pull/2753), and having
multiple implementations of the network layer made it much more
difficult to change some of the code.
Store shuffle data in-memory via the block mgr.

As-is, shuffle data will never be deleted from the block
manager (need to add cleaning to do this).
Some fixes to metrics
Fixed problem with mis-handling of 0-byte shuffles
Removed sort-based shuffle and disk spilling.

This commit removes the sort-based shuffle, including
all associated unit tests.  It also removes disk spilling
that happens through the external sorter. This spilling was
used primarily as part of the sort-based shuffle, and is not
compatible with the monotasks execution model.