benchmark

Cache PerfCounters instance in PerfCountersMeasurement (#1308)

This patch fixes #1306, by reducing the pinned instances of
PerfCounters.

The issue is caused by creating multiple pinned events in the
same thread, doing so results in the Snapshot(PerfCounterValues* values)
failing, and that's now discoverable.
Creating multile pinned events is an unsupported behavior currently.
The error would be detected at read() time, not
perf_event_open() / iotcl() time.

The unsupported benavior above is confirmed by Stephane Eranian @seranian,
and he also pointed the dectection method.

Finished this patch under the guidance of Mircea Trofin @mtrofin.