Fix instruction count for start_measuring_time

[travisdowns]

Currently perf_tests::start_measuring_time (and the corresponding stop call) doesn't correctly measure the instruction count. It always reports 0 instructions if any of these methods is used in a perf test.

To fix this, we capture the linux_perf_event pointer in the time_measurement class.

[travisdowns]

@bhalevy this may also be in your wheelhouse.

[travisdowns]

Thanks for the review @bhalevy, much appreciated.