client: close mds sessions in shutdown()

client: close mds sessions in shutdown()

Usually this happens in unmount(), but when we
have instantiated Client without mounting (to
send MDS commands), we need to handle closing
any open sessions in shutdown as well.

This is the correct replacement for the mark_down()
call that was removed from handle_command_reply
in the last commit.

Signed-off-by: John Spray <john.spray@redhat.com>