Allow shutdown after timeout

If a timeout has occurred, the send to recv in recvICMP would block forever
because no goroutine was receiving on that channel anymore. This leaves
behind 2 goroutines for each timeout: 1 trying to send an recv, and another
waiting for the wait group. Instead, used a buffered channel so that the
sender can send even with no listener. This cleans up the goroutines and
collects the channel.