I find myself often wishing for or manually adding both a timestamp and pid:tid prefix for verbose messages. I think this is a generally useful thing and would like to add it. Any objections?

I have several questions relating to this:

• Should it just be for 'msg_event' messages (i.e. verbose) or all message types? Just verbose works well enough for debugging convenience. Having all types have the same prefixes seems desirable too.
• Other than a comment in __eventv, the structure of a message seems like it is not documented and this wouldn't break things. Did I miss it?
• The comment in __eventv separates things with a comma. Do we expect people are parsing these as comma-separated values? This should probably follow suit.
• What should it look like? Any preferences? Two obvious candidates would be:
  sec:usec, pid:tid, session->error_prefix, ...
  [sec:usec], [pid:tid], session->error_prefix, ...