Use timer for background PINGs instead of a long-live goroutine

[rueian]

Currently, we use a long-live goroutine for each connection to do background PINGs:

rueidis/pipe.go

Lines 325 to 327 in 3171947

	if p.timeout > 0 && p.pinggap > 0 {
	go p.backgroundPing()
	}

In the current approach, we create the same amount of background goroutines as the number of connections that wait periodically to send a PING to the connection. I’m curious to know if using time.AfterFunc instead would enhance memory usage and improve the scheduling for other more important goroutines.