[suggestion] Have 'Network Id' sent with all requests

[yurivict]

In the situation when somebody wants to have entirely their own network not interacting with the default Tox network, it is good to have 'Network Id'. This id should be set in all clients and bootstrap nodes, sent with all requests, and mismatched packets should be dismissed.

This can be useful in 2 situations:

• Some group of people wants to have their private network completely disconnected from the regular, default network.
• While testing of some new feature, it is great to be able to not worry about accidental interaction with some other peers.

The 'Network Id' must be very easy to implement in the new protocol version.

[iphydf]

I like this idea. I've wanted a "test network" before (#246), but this more general solution appeals to me.