A better way to detect senders on Local network

[plateaukao]

plateaukao@6e26952

This is a commit to replace original process of ping a lot of IP addresses by using UDP multicast mechanism.
It's much faster for finding senders (within 1 second), and no need to use wait until timeout, neither using compute for an extra isolate.

[plateaukao]

Another suggestion is that: after using UDP multicast, the sharik.json content can actually be sent out in the UDP datagram directly, there's no need to try the get sharik json step. This also saves time and reduce the code logic complexity.