Rate limit based on trigger.time (if available)

[dgw]

Added in sopel 6.2.1, so obviously depends on #2 (my bot's running 6.x since about a week ago).

Using trigger.time means rate-limiting will use the value of server-time if available. No real change, but should make the rate limit more accurate (not that anyone ever complains about it being off by a second or two).

[dgw]

I'm no longer positive that I want to do this. Messed with it a bit in a WIP branch before starting on #42, and it seemed like a lot of code uglification (new parameters to pass around) for not much practical gain. Plus, trigger.time is now an aware datetime in Sopel 8, which complicates things significantly in terms of storing/retrieving rep_used.