iproto: fix accepting new connections after shutdown is started

When shutdown is started iproto stop accepting new connections yet new
connections are still possible. One can use box.session.new() or use
box.cfg{} to start accepting connections again. This is a minor bug but
in the next patch we are going to free all iproto resources properly and
rely on the fact that we stop accepting new connections. (Before
iproto_free we call iproto_drop_connections to make sure all connections
are finished. But if we continue to accept new connections then
connection that established after dropping may not be finished.)

Part of #8423

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