Upgrade reactor to Californium-RELEASE

[chang-chao]

Spring 5.1 and Spring Boot 2.1 uses Californium-RELEASE, which comes with breaking changes in its APIs, but also a change in the package names and artifact coordinates from Bismuth-SR11.
So currently we can't use Spring 5.1 and Spring Boot 2.1 with rsocket-java, which currently uses reactor Bismuth-SR11.

[yschimke]

We've been pretty aggressive with upgrades, although we are reliant on reactor-netty, is that part of the same release train?

cc @nebhale @rdegnan any objections to a quick upgrade here?

[chang-chao]

By the artifact coordinates change above , I mean, the group name of the reactor-netty artifact was changed from io.projectreactor.ipc (version 0.7.x used in Bismuth-SR11 release train) to io.projectreactor.netty(version 0.8.0.RELEASE which is used in the Californium-RELEASE train.)

[nebhale]

There are some minor regressions near Processor but probably won't be an issue. reactor-netty's API has changed pretty significantly, so you'll need to handle that, but you can use R2DBC's migration as an example. Beyond that, ensure that the tests run and it should be a no-brainer improvement.