fix bug where DataLayerPrefetch creates its own Caffe singleton, causing
the phase to always be set to TRAIN (always random crops) and RNG failures
do the same as prev commit for ImageDataLayer
Merge pull request #348 from jeffdonahue/datalayer-singleton-bugfix

Fix singleton call in data layers

Note previous crash reports were not in fact due to RNG, although they led to improvements of the RNG code.