Revert of rebaseline_server: multithreaded loading/diffing of images (https://coderev...

Revert of rebaseline_server: multithreaded loading/diffing of images (https://codereview.chromium.org/235923002/)

Reason for revert:
Caused https://code.google.com/p/skia/issues/detail?id=2423 ('"Deadline exceeded" error when connecting to live rebaseline_server').  Reverting until I can figure it out.

Original issue's description:
> rebaseline_server: multithreaded loading/diffing of images
>
> BUG=skia:2414
> NOTRY=True
>
> Committed: http://code.google.com/p/skia/source/detail?r=14184

R=rmistry@google.com
TBR=rmistry@google.com
NOTREECHECKS=true
NOTRY=true
BUG=skia:2414

Author: epoger@google.com

Review URL: https://codereview.chromium.org/239623002

git-svn-id: http://skia.googlecode.com/svn/trunk@14207 2bbb7eff-a529-9590-31e7-b0007b416f81