Timeout used by park_timeout seems way too short, causes heavy CPU use

I have a small test program (implementing a component of a real application) using Bus that was idling at about 100-110% CPU utilization (1+ cores of 8), which is excessive.

When I changed the timeouts used by the two calls to `park_timeout` to be 1ms (`Duration::from_millis(1)`) rather than 1us currently in the official release, the CPU utilization under a pretty realistic but on the heavy side real-world load (not just idle, and including some computation) fell to about 3.5%.

I think this 3.5% CPU utilization is pretty impressive. Nice library you've got there :-)