Fix xio_connection teardown.

The XIO_SESSION_CONNECTION_DISCONNECTION event indicates a
connection is no longer useable, but not that it can be destroyed.

Give XioConnection an atomic connected property, and set it to
false on remote disconnection.  Also xio_connection_destroy() is
now called in XioConnection's destructor.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>