Fix bug in fb.util.timing: start should not reset patterns
Fix bug in fb.util.timing: start should not reset patterns

Summary:
You should be able to run start() / finish() as many times as you want on
the same set of patterns.

Test Plan: ran trace_test.llar, the output looks good

Reviewed By: kma@fb.com

Subscribers: tulloch, soumith, pamelavagata

FB internal diff: D1698086

Signature: t1:1698086:1416607574:39695bdbff2bf5a31e71db8b72e6c516fb691b5d

raphaelamorim/fblualib,facebook/fblualib,raphaelamorim/fblualib,szagoruyko/fblualib,szagoruyko/fblualib,soumith/fblualib,facebook/fblualib,soumith/fblualib