Use chrono for timing

Summary: Since we depend on C++11, we might as well use it for timing, instead of this platform-depended code.

Test Plan: Ran autovector_test, which reports time and confirmed that output is similar to master

Reviewers: ljin, sdong, yhchiang, rven, dhruba

Reviewed By: dhruba

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D25587