Don't panic when cancelling an unknown consumer

[michaelklishin]

See https://github.com/spring-projects/spring-integration/issues/2658#issuecomment-474452945 for background. Kudos to @bojanv55 for solid investigative work.

When a user tries to cancel a consumer or when this client receives a basic.cancel for a consumer it doesn't recognize, instead of throwing an IOException it should log a warning and ignore the method. This is what works well for other clients.

Note that such scenarios are realistic and have to do with inherently racy consumer state management on server and client ends. Being more defensive and not panicking is the right thing to do.

Per discussion with @acogoluegnes.

[michaelklishin]

@acogoluegnes and I decided to consider making Channel#basicCancel idempotent for 6.0 but avoid throwing an exception for server-sent basic.cancel frames.