Proper TimeUnit support for Bukkit tasks

Bukkit doesn't support `TimeUnit` out of the box, so we currently convert the time into ticks, and go from there. Turns out, we can use [`TimerTask`](https://docs.oracle.com/javase/8/docs/api/java/util/TimerTask.html) to schedule repeating tasks async, but it might not integrate very nicely...

More investigation is required.

Start by tracing how the library currently converts TimeUnit values into Bukkit ticks for scheduled tasks. Investigate whether TimerTask can provide repeating asynchronous scheduling without harming integration, and consider the work complete when proper TimeUnit support is defined and the scheduling approach is confirmed to integrate cleanly.

Written by the indexing model from the issue text.