Adds cached values of the 5 top-level headers to the metrace_stream_t interface and implements this for the readers.
Adds checks of these values to invariant_checker.
Adds a test with skipped instructions using the skip_unit_tests checked-in trace.
Reverses the order of the initial 2 markers and the tid,pid pair sent to the reader to avoid a 0 tid in tools. I am surprised this hasn't caused more problems and I thought it was already this fixed way.
Issue: #5538