Add better toString implementations to Session classes

Whenever something goes wrong with a Session instance, the session is often logged. The {{toString}} of some sessions is missing, or does not provide to much helpful information, leading to log lines like this:

2019.08.23 19:19:05 WARN [Server SR - 364503950]: org.jivesoftware.openfire.net.BlockingReadingMode - Stream error detected. Session: org.jivesoftware.openfire.session.LocalIncomingServerSession@133c2019 status: 1 address: igniterealtime.org/10dg6uzpgr id: 10dg6uzpgr \

More context can be provided easily, by exposing that through to the toString method.