make shutdown timeout configurable

this patch makes the shutdown timeout configurable and sets the default timeout to 60 seconds, so server processes which take a few seconds to shutdown gracefully get a chance to do so
Use a duration flag instead (`f.DurationVarP`). Also sanity check this: if it's 0 or negative, reject. (I don't remember whether a duration flag can be negative; you'll have to check.)

Also, after you've made that change and updated the flag description, rebuild and run `reflex -h` and update the appropriate section of the README.