Moves the timestamp+cpu headers to be added at buffer creation time instead of at buffer output time.
Makes instru_t::frozen_timestamp_ use release-acquire semantics and sets it for -align_endpoints at detach time.
Adds a 'minimum timestamp' recorded at post-attach time. At buffer output time, we examine the buffer's timestamp and if it was from before the minimum, we replace it with the current (not the minimum) timestamp.
Tested on a large app where these changes result in much tighter timestamps matching the burst period.
Issue: #2039 (closed)