Ping: repeat requests periodically #415
Description
Currently, ping request is only sent once when a connection is established. As a mechanism of detecting broken connections ping only makes sense if it is repeated periodically. For example, libp2p repeats pings every 15 seconds by default. We need to do the same.
Care should be taken for ping substreams to not keep connections alive if there are no other substreams. See #414 for more info.