cbus: add a single idle timer, chain up coro_transfer in fiber pool

- don't use fiber_yield_timeout, it's too slow; use a single
idle timer and invoke the fiber which hasn't been invoked for longest
period so that it can time out

- chain up the running fibers in a pool so that coro_transfer()
goes to the next fiber, not to the sched fiber, most of the time

This together improves cbus speed and reduces CPU spent in
coro_transfer().