Missing code to cleanup worker state.

..also it turns out vmap->decref() in ~proc is not what I wanted.

Previously, sched always paused call tracing but if it then returned
directly because there was nothing else to schedule, call tracing
didn't get resumed.  We could simply resume tracing in this case, but
this fix instead moves the pause *after* the next proc is picked, so
that we don't get lots of spurious pause/resume pairs.