Caffe2: Add Open method to DBReader which takes DB pointer
Caffe2: Add Open method to DBReader which takes DB pointer

Summary: Currently the DBReader always creates the DB instance itself when Open is called.  Add an Open method that takes in a DB pointer and takes ownership of it, so the DB can be initialized outside the DBReader.

Reviewed By: panshen1

Differential Revision: D5392458

fbshipit-source-id: d8660ab41d349f32030e4934b47bd17256a440df

bwasti/caffe2,pietern/caffe2,pietern/caffe2,xzturn/caffe2,bwasti/caffe2,xzturn/caffe2,xzturn/caffe2,bwasti/caffe2,sf-wind/caffe2,davinwang/caffe2,davinwang/caffe2,bwasti/caffe2,xzturn/caffe2,xzturn/caffe2,sf-wind/caffe2,Yangqing/caffe2,davinwang/caffe2,Yangqing/caffe2,sf-wind/caffe2,Yangqing/caffe2,pietern/caffe2,davinwang/caffe2,sf-wind/caffe2,pietern/caffe2,pietern/caffe2,davinwang/caffe2,caffe2/caffe2,bwasti/caffe2,Yangqing/caffe2,sf-wind/caffe2,Yangqing/caffe2