remove time.clock() usage
remove time.clock() usage

Summary:
To get ready for python 3.8 where `time.clock()` got removed

```
$ python3
Python 3.7.5+ (default, Jan 24 2020, 11:19:43)
[GCC 7.x 20200121 (Facebook) 8.x] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import time
>>> time.clock()
__main__:1: DeprecationWarning: time.clock has been deprecated in Python 3.3 and will be removed from Python 3.8: use time.perf_counter or time.process_time instead
0.017866
```

Reviewed By: yfeldblum

Differential Revision: D21289058

fbshipit-source-id: 999210c1e05ca71fa4585e2a10795ff75ba9bb84

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift