fix: accept two incoming PING streams per peer

[bajtos]

Modify the default configuration for the PING protocol to accept at most two streams per peer, as recommended in the PING protocol spec.

Quoting https://github.com/libp2p/specs/blob/d4b5fb0152a6bb86cfd9ea/ping/ping.md?plain=1#L38-L43

The dialing peer MUST NOT keep more than one outbound stream for the ping protocol per peer. The listening peer SHOULD accept at most two streams per peer since cross-stream behavior is non-linear and stream writes occur asynchronously. The listening peer may perceive the dialing peer closing and opening the wrong streams (for instance, closing stream B and opening stream A even though the dialing peer is opening stream B and closing stream A).

[achingbrain]

Thanks!