Add execution epoch to ParSeq trace

Epoch in this context means sequence of tasks executed without thread switch.
This will allow analysis of various improvements e.g. benefit of auto batching, tasks fusing, executor improvements, queue implementations etc.

Start by locating ParSeq's trace implementation and the execution path where tasks may switch threads. Read how trace events are represented and how task execution is scheduled, then determine how an epoch should be recorded when tasks run without a thread switch. Done means traces expose execution epochs in a form that supports the analysis described in the issue.

Written by the indexing model from the issue text.