libp2p-ping 0.22.0

Ping protocol for libp2p
Documentation
# 0.22.0 [2020-09-09]

- Update `libp2p-swarm` and `libp2p-core`.

# 0.21.0 [2020-08-18]

- Refactor the ping protocol for conformity by (re)using
a single substream for outbound pings, addressing
[#1601]https://github.com/libp2p/rust-libp2p/issues/1601.

- Bump `libp2p-core` and `libp2p-swarm` dependencies.

# 0.20.0 [2020-07-01]

- Updated dependencies.

# 0.19.3 [2020-06-22]

- Updated dependencies.

# 0.19.2 [2020-06-18]

- Close substream in inbound upgrade
  [PR 1606]https://github.com/libp2p/rust-libp2p/pull/1606.