Explore Backpressurable#getCapacity optimizations

[smaldini]

Backpressurable#getCapacity can be a useful tool to detect if the source Publisher is a Completable (0), Single (1), Unbounded (Long.MAX), Bounded (N > 1 && < Integer.MAX) or Mixed (-1) backpressure strategy.

Some operator might adapt their supplied Queue while other can choose to short-circuit more expensive inner Subscriber.