Issue #24900 add a few unit tests for ManagedConnectionImpl and ConnectionHolder to understand associateConnection logic

[escay]

I wrote a few unit tests to start to understand associateConnection logic which is used by ConnectorXAResource in connector-runtime. Added some javadoc to understand ManagedConnectionImpl parameters. Added a listener null check and added unregister listener logic which seemed to be missing. Removed a boolean parameter from setActive(true) which was always true.

[dmatej]

Sorry, we had a broken metadata file in maven cache, that crashed your build, I have restarted the job now