Populate recent time once before starting upkeep thread.

[tobz]

During some testing in #92, it was discovered that the initial call to set recent time can take unexpectedly long, which leaves the initial recent time at 0, which can lead to the first delta attempt using recent time (i.e. let start = Clock::recent(); ...; let delta = start.elapsed()) being unexpectedly large.

We should do a single call to populate recent time before we spawn the actual upkeep thread, so that we're anchored as close as we possibly can be to the true time, rather than just having the initial value of zero.