Move set_stream_id() to protected field.
Move set_stream_id() to protected field.

Summary:
This does not change any existing code behavior - as part of the event
abstractions, this is a cautious step to reduce the interfaces exposed
from contexts. Nothing else is changing.

Reviewed By: harouwu

Differential Revision: D5656597

fbshipit-source-id: 53c5caf278613e610daf6ad3ca4bb6da73367cfc

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