When `DirectChannel` is used for reply producing, the data is
handled on the same thread which has produced it (normally), so
if we have a request-reply afterwards (e.g. `gateway()`), this thread
is blocked waiting for reply.
When the thread is assumed to be non-blocked (e.g. Netty event loop),
the request-reply withing such a thread for the same non-blocking client
causes a deadlock: the thread waits for reply, but at the same time it
supposes to fulfil a synchronization barrier with that reply
* Fix `AbstractMessageProducingHandler.asyncNonReactiveReply()` to use
a `publishOn(Schedulers.boundedElastic())` for reply `Mono` to free
producing thread from potential downstream blocking
* Demonstrate deadlock with a new test in the `RSocketDslTests`;
the original report was against WebFlux, but conditions are really
the same: `reactor-netty` is used as a low-level client
**Cherry-pick to `5.4.x`**
0f2285d5a3