Document that listeners should not block in message or channel/connectionstate listeners

[paddybyers]

trafficstars

Listeners are called by specific threads in the library - eg the ConnectionManager thread or the WSClient reader thread - and these can't be blocked. The documentation should make this clear.

┆Issue is synchronized with this Jira Task by Unito

[cesartl]

Ideally, this would be explained in both online documentation and the Javadoc on the subscribe method.