Refactoring of PerfCounters infrastructure

[HFTrader]

The main feature in this pull request is the removal of the static sharing of PerfCounters and instead creating them at the top RunBenchmarks() function where all benchmark runners are created. A single PerfCountersMeasurement object is created and then shared with all the new BenchmarkRunners objects, one per existing benchmark.

Other features conflated here in this PR are:

• Added BENCHMARK_DONT_OPTIMIZE macro in global scope
• Removal of the IsValid() query, being replaced by checking the number of remaining counters after validity tests
• Refactoring of all GTests to reflect the changes and new semantics
• extra comments throughout the new code to clarify intent
• fix a harmless issue where the Create() method would invalidate the entire set of counters if one of them failed to open after being validated

It was truly impossible to separate all those features in different PRs as requested since they are so interdependent on each other so I'm just pushing them together and asking for forgiveness.

This PR comes replacing PRs 1555 and 1558 which have been closed.

[HFTrader]

The Bazel failures dont seem related to code. What to do?

[dmah42]

rerunning the osx one, that looks like a local failure. the windows one seems like some timings changed away from expectations. that shouldn't be related to this PR. i'l try rerunning that too.

[mtrofin]

lgtm, some nits

[HFTrader]

rerunning the osx one, that looks like a local failure. the windows one seems like some timings changed away from expectations. that shouldn't be related to this PR. i'l try rerunning that too.

Maybe because some tests are now running in full while before they were being collapsed by the compiler?

[dmah42]

thanks for all the iterations :D