TimeAlarm duration argument for simpler code without precision degradation

[kevinjwalters]

The new alarm.time.TimeAlarm() function from #3767 takes two forms of absolute time as an argument. The 30bit fp number representation of time from time.monotonic() loses precision over time and this will impact users who use monotonic_time keyword argument. For most users a relative time will be easier to use and avoids this problem as the library can deal with maintaining best precision under the covers.

The example code:

pause = alarm.time.TimeAlarm(monotonic_time=time.monotonic() + 20)

would become:

pause = alarm.time.TimeAlarm(relative_time=20)

There may be better names for that new kwarg...

[tannewt]

My preference is for us to reset monotonic every time the VM resets. That will mitigate a lot of the problems with it.