Add option to automagicaly synchronize pt-heartbeat

When i run pt-heartbeat --update and pt-heartbeat --monitor, i have to make sure, they run synchronized, e.g.: updating and checking run at about the same time. When adding an option which waits " interval - (unix_timestamp % interval) " before the first update/check, all instances of pt-heartbeat should run synchronized (given that the clocks on all involved machines are synchronized -> ntp).