The samba3.blackbox.durable_v2_delay test triggers
a race where the first connection with a specific client_guid
already entered exit_server_common(), but didn't left it
before the 2nd connection with the same client_guid is accepted.

The smbd child for the 2nd connection found the smbXsrv_client record
of the first connection and used smb2srv_client_connection_pass()
in order to send the new connection over, but it doesn't wait
for any kind of ack, that the connection was actually taken over.

We need an MSG_SMBXSRV_CONNECTION_ACCEPTED as response to
MSG_SMBXSRV_CONNECTION_PASS.

After calling smb2srv_client_connection_pass(), the sending process
needs to watch the smbXsrv_client.tdb record and wait for a
MSG_SMBXSRV_CONNECTION_ACCEPTED message.

If we get MSG_SMBXSRV_CONNECTION_ACCEPTED we can exit the new process.

If the smbXsrv_client record got removed, the new process should go on
and take care of the new connection, as if it would have been the
first connection for that client guid.