BCO sometimes blocks forever during shutdown.

[DivineThreepwood]

Because of issue https://github.com/open-rsx/rsb-java/issues/28 the participants do not properly deactivate their spread connection and block the shutdown routine. Additionally because of issue open-rsx/rsb-java#32 some internal routines get not directly informed about the deactivation and still continue their work in case of heavy operations.

Therefore, issue is blocked by:

• https://github.com/open-rsx/rsb-java/issues/28
• Async send future is not canceled after remote server is deactivated. open-rsx/rsb-java#32