Rate limit remaining-time placeholders contain microseconds

[dgw]

Tin, pretty much:

[09:06] <notdgw> .test
[09:06] -SopelTest- Relax, notdgw. You can .test again in 0:04:58.030534.

{time_left} and {time_left_sec} don't need this much precision. Effectively doing math.ceil() to the next second would be good enough. (Truncating the microseconds would probably be good enough, too, but I'd prefer just always rounding up because users will try to snipe the remaining time if it's from a competitive game or something.)

Would be nice to include in 8.0.2 but not critical; can bump to a new 8.0.3 milestone if it's holding up the line.