Mono.toProcessor to automatically connect

[simonbasle]

Since #566, Mono.subscribe() returns a Disposable and the MonoProcessor is available via Mono.toProcessor() instead. However, the old style would allow to create a processor, connect to it and chain further operators in one line:

Mono.cache().subscribe().anotherOperator

What is missing with toProcessor() is the ability to connect the MonoProcessor then chain operators (.toProcessor().subscribe() doesn't allow the later).

We could add a toProcessor(boolean) variant that optionally connects, or a toConnectedProcessor() that automatically connects.

[simonbasle]

after discussing it with @smaldini, no new operator/variant, but toProcessor() will connect()/subscribe().