bitforce: Minimally refactor to adapt to new minerloop_async
stale_work_future function to determine in advance if a work/share will be stale at some future time
bitforce: Use poll device API when job_start needs to wait
bitforce: Use poll device API when job_get_results needs to wait
Bugfix: bitforce: Avoid polling continuously between work restart and job completion
Bugfix: Use waddstr instead of wprintw to display completed device summary line, so literal %s don't get interpreted as formatting options