Use NettyChannelBuilder to enable blocking calls in callbacks

[cbornet]

It's currently not possible to do blocking calls in listener handlers while preserving back-pressure as that blocks the vertx event loop (see #1089 #1285).
Instead of VertxChannelBuilder, we can use NettyChannelBuilder delegates onNext/onError/onCompleted to a thread-pool with ordering guarantees.
It is OK to do it as the vertx-grpc stubs are just wrappers around the grpc-java ones and don't actually use the vertx loop. So they are compatible with any ManagedChannelBuilder (eg. in unit tests InProcessChannelBuilder is used).

Alternatives:

• Rejected: pass responses to an ordered blocking call executor. This solves the blocking issue but breaks the back-pressure chain (see WIP: Support blocking calls in client response handlers #1558 )
• Future work: use the vertx-grpc-client (Switch to vertx-grpc-client #1372)

[k8s-ci-robot]

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