feat(timestamps): more accurate timestamps

[JonasBa]

We were previously not collecting accurate timestamps in regards to when profiling was started/stopped relative to the transaction.

Since calling profile.GetStartTime() and profile->GetEndTime() do not return relative timestamps it is not suitable to use that data to determine the elapsed time between the start of transaction. Having a relative ts here would improve accuracy. For those reasons, we use process.hrtime to mark the start/stop timestamps of calls to startProfiling and stopProfiling.

We error on the safe side by placing the hrtime marker call after startProfiling which effectively means that our relative start time will be reported higher than it is, however this is beneficial as it allows us to monitor the delay to start/stop profiling which can be different depending on the v8::CpuProfilingLoggingMode used.

process.hrtime markers are added to following points in the transaction/profiling lifecycle.

• after startTransaction
• after startProfiling
• after stopProfiling

[JonasBa]

Oh and I removed /lib because it shouldn't have been there in the first place.