Consider implementing riptide based on a reactive stack

[whiskeysierra]

Spring uses reactor as it's abstraction, but in the context of #837 we shouldn't be bound by that.

[whiskeysierra]

Returning a CompletableFuture is technically enough, but our current use of InputStream requires either blocking calls or at least buffering in case non-blocking IO. Neither of which is ideal when working reactive.

[whiskeysierra]

The AsyncRestTemplate also supports non-blocking interactions. The main difference is it can’t support non-blocking streaming, like for example Twitter one, because fundamentally it’s still based and relies on InputStream and OutputStream.

https://docs.spring.io/spring/docs/5.0.0.M4/spring-framework-reference/html/web-reactive.html#web-reactive-client