periodic/periodic.go
func Ticker(ticker <-chan time.Time, procfunc gp.ProcessFunc) gp.Process {
	return gp.Go(callOnTicker(ticker, procfunc))
}
func Ticker(ticker <-chan time.Time, procfunc gp.ProcessFunc) gp.Process {
	return gp.Go(callOnTicker(ticker, procfunc))
}
// Ticker calls the given ProcessFunc every time the ticker fires.
// This is sequentially rate limited, only one call will be in-flight at a time.