iproto: fix multiple socket closures and not deleting unix socket path

All iproto threads listening same socket, and if user change listen
address, this socket is closed in each iproto thread. This patch fix
this error, now socket is closed only in main thread, and in other threads
we are only stop listening, without socket closing. Also this patch fix
error, related to the fact, that tarantool did not delete the unix socket
path, when it's finishing work.