display spans for a certain time interval
display spans for a certain time interval

Summary:
This diff adds a couple of options to `htrace_to_chrome.py` so that users can specify start and end timestamps for displaying spans.
For example, the arguments `--start_time x  --end_time y` indicate that spans that finish before `y` or start after `x` will not be included in the final chrome tracing json file.

This also adds timestamp information to the spans which can serve as hints to the command line argument values.

Differential Revision: D4372220

fbshipit-source-id: a2b0af3be6861448874d804b30426df1b67a676e

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