[SPARK-6627] Some clean-up in shuffle code.

Before diving into review #4450 I did a look through the existing shuffle
code to learn how it works. Unfortunately, there are some very
confusing things in this code. This patch makes a few small changes
to simplify things. It is not easily to concisely describe the changes
because of how convoluted the issues were, but they are fairly small
logically:

1. There is a trait named `ShuffleBlockManager` that only deals with
   one logical function which is retrieving shuffle block data given shuffle
   block coordinates. This trait has two implementors FileShuffleBlockManager
   and IndexShuffleBlockManager. Confusingly the vast majority of those
   implementations have nothing to do with this particular functionality.
   So I've renamed the trait to ShuffleBlockResolver and documented it.
2. The aforementioned trait had two almost identical methods, for no good
   reason. I removed one method (getBytes) and modified callers to use the
   other one. I think the behavior is preserved in all cases.
3. The sort shuffle code uses an identifier "0" in the reduce slot of a
   BlockID as a placeholder. I made it into a constant since it needs to
   be consistent across multiple places.

I think for (3) there is actually a better solution that would avoid the
need to do this type of workaround/hack in the first place, but it's more
complex so I'm punting it for now.

Author: Patrick Wendell <patrick@databricks.com>

Closes #5286 from pwendell/cleanup and squashes the following commits:

c71fbc7 [Patrick Wendell] Open interface back up for testing
f36edd5 [Patrick Wendell] Code review feedback
d1c0494 [Patrick Wendell] Style fix
a406079 [Patrick Wendell] [HOTFIX] Some clean-up in shuffle code.