Support http/2 configuration with Reactor-Netty

[bclozel]

Reactor-Netty 0.8 now supports the http/2 protocol (and h2c transparently, by default).
We should update the server configuration to reflect that - waiting for reactor/reactor-netty#370 to be looked at first.

Then we can reflect that in the documentation, including the various supported setups (JDK version, native library support for TLS, etc). The constraints and supported setups should be quite similar to Jetty and Tomcat; except that the native library of choice is "io.netty:netty-tcnative-boringssl-static".

[bclozel]

Waiting for reactor/reactor-netty#384