libp2p-ping improvements. (#1049)

* libp2p-ping improvements.

  * re #950: Removes use of the `OneShotHandler`, but still sending each
    ping over a new substream, as seems to be intentional since #828.

  * re #842: Adds an integration test that exercises the ping behaviour through
    a Swarm, requiring the RTT to be below a threshold. This requires disabling
    Nagle's algorithm as it can interact badly with delayed ACKs (and has been
    observed to do so in the context of the new ping example and integration test).

  * re #864: Control of the inbound and outbound (sub)stream protocol upgrade
    timeouts has been moved from the `NodeHandlerWrapperBuilder` to the
    `ProtocolsHandler`. That may also alleviate the need for a custom timeout
    on an `OutboundSubstreamRequest` as a `ProtocolsHandler` is now free to
    adjust these timeouts over time.

Other changes:

  * A new ping example.
  * Documentation improvements.

* More documentation improvements.

* Add PingPolicy and ensure no event is dropped.

* Remove inbound_timeout/outbound_timeout.

As per review comment, the inbound timeout is now configured
as part of the `listen_protocol` and the outbound timeout as
part of the `OutboundSubstreamRequest`.

* Simplify and generalise.

Generalise `ListenProtocol` to `SubstreamProtocol`, reusing it in
the context of `ProtocolsHandlerEvent::OutboundSubstreamRequest`.

* Doc comments for SubstreamProtocol.

* Adapt to changes in master.

* Relax upper bound for ping integration test rtt.

For "slow" CI build machines?

src/lib.rs

@@ -283,8 +283,8 @@ impl CommonTransport {
     #[inline]
     #[cfg(not(any(target_os = "emscripten", target_os = "unknown")))]
     pub fn new() -> CommonTransport {
-        let transport = tcp::TcpConfig::new();
-        let transport = dns::DnsConfig::new(transport);
+        let tcp = tcp::TcpConfig::new().nodelay(true);
+        let transport = dns::DnsConfig::new(tcp);
         #[cfg(feature = "libp2p-websocket")]
         let transport = {
             let trans_clone = transport.clone();