net.box: fix hang in graceful shutdown protocol

The graceful shutdown protocol works as follows:

 1. The server sends a shutdown request (the box.shutdown event) to all
    its clients that subscribed to it.
 2. Upon receiving a shutdown request, a client is supposed to close its
    connection.
 3. The server waits for all clients subscribed to box.shutdown event to
    exit.
 4. The server exits.

In net.box, the box.shutdown event is processed by `remote._callback`.
The problem is it may occur that `remote._callback` is garbage collected
while the `remote` object isn't. If this happens, the shutdown request
will never get processed, and the server won't exit until the `remote`
object is garbage collected, which may take forever.

Let's fix this issue by breaking the worker loop if we see that the
callback was garbage collected.

Closes #7225

NO_DOC=bug fix