Improve broker relay's shutdown and availability events

Previously, when the broker relay was shut down, the TCP client was
closed and the relay sessions were left to find out about the
shutdown as a result of their TCP connections being closed. This led
to problems where an attempt could be made to use a session that was,
in fact, in the process of being shut down.

This commit updates the broker relay to explicitly close each of its
relay sessions as part of its stop processing.
As part of the broker relay being shut down explicitly close each of
its relay sessions. It does so before closing the TCP client so that
the relay sessions know that they are  disconnected before their TCP
connections are closed.

The broker relay's publishing of availability events has also been
improved. Prior to this commit, availability events were published
based on the availability of any relay session. For example, this
meant that a successfully established client relay session would
result in an event being published indicating that the broker's
available even if the system relay session was yet to be established.
This commit updates the relay so that broker availability events are
only published by the system relay session. This allows application
code the use these events as an accurate indication of the
availability of the broker. Clients that are interested in the
broker's availability can find out through the use of heart beats or
through the receipt of an ERROR frame in response to an attempt to
communnicate with the broker.