Change App#refresh_loop sleep to App::REFRESH_SUBINTERVAL 0.1s .

Previously, `App#refresh_loop` slept `1s` at a time. Extract subinterval
to `App::REFRESH_SUBINTERVAL`, setting to `0.1s`. Since the threads for
`App#command_loop` and `App#refresh_loop` are joined, this has the effect
of faster shutdown (considering maximum wait).