benchmark

Make `PauseTiming()` and `ResumeTiming()` per thread. (#286)

* Change to using per-thread timers

* fix bad assertions

* fix copy paste error on windows

* Fix thread safety annotations

* Make null-log thread safe

* remove remaining globals

* use chrono for walltime since it is thread safe

* consolidate timer functions

* Add missing ctime include

* Rename to be consistent with Google style

* Format patch using clang-format

* cleanup -Wthread-safety configuration

* Don't trust _POSIX_FEATURE macros because OS X lies.

* Fix OS X thread timings

* attempt to fix mingw build

* Attempt to make mingw work again

* Revert old mingw workaround

* improve diagnostics

* Drastically improve OS X measurements

* Use average real time instead of max