Add EPOCH_TIMESTAMP conditional compile flag

[eschreiber-alink]

Add EPOCH_TIMESTAMP conditional compile flag. Changes mgen log format to use a decimal timestamp using timeval fields in format of tv_sec.tv_usec. The format avoids midnight rollover. The compile flag is off by default preserving the existing timestamp format.

[bebopagogo]

I also think this might be useful as a runtime option, too. One thing to note is the existing binary log format option does already provide a full Unix timestamp for each event.

[eschreiber-alink]

Agreed that it's preferable as a runtime option. I'll see if I can rework it that way.

Thanks for the tip on the binary log format - I wasn't aware.