Timer: handle timeout correctly

[vtjnash]

I am not sure why we ever used round+1 instead of ceil+1, as this is
strictly more correct.

[IanButterworth]

Could Timer support a reset(::Timer) method?

That way interval creep could be addressed by something like

Timer(0; interval = 5) do t
    foo()
    Dates.second(Dates.now()) % 5 != 0 && reset(t)
end

[vtjnash]

What does reset do?

[IanButterworth]

Reset the timing as if the timer had just been recreated.

[IanButterworth]

It could error if the timer is closed