* Add support for the `MetadataExtractor` in the
`AbstractRSocketConnector`, `IntegrationRSocketMessageHandler`
and `IntegrationRSocket` for more flexible RSocket metadata
* Use `DefaultMetadataExtractor` in the
`IntegrationRSocketMessageHandler.afterPropertiesSet()` since the getter
is not available on the super class yet
* Use a `MetadataExtractor` in the `IntegrationRSocket` instead of an
old `CompositeMetadata` manipulation
* Use `PayloadUtils.retainDataAndReleasePayload()` since it is `public`
already
* Downgrade to the `rsocketVersion = '0.12.2-RC4'` since it looks like
the latest `1.0.0-RC1-SNAPSHOT` doesn't parse a frame properly on the
server side
c6a5ea874c