[WFCORE-19] properly cancel active mgmt operations on channel close

https://issues.jboss.org/browse/WFCORE-19
A comment on how this is expected to be used would be good. Searching for usages, I see it's how HttpShutdownService gets notified of shutdown. It's kind of a pain to have to do that kind of search when trying to understand how complex code works. Doesn't need to be javadoc; could just be a simple code comment.