Add limits to the number of peers connected to per subnet

[pixelspark]

What Bitcoin does against sybil attacks:

Bitcoin makes these attacks more difficult by only making an outbound connection to one IP address per /16 (x.y.0.0). Incoming connections are unlimited and unregulated, but this is generally only a problem in the anonymity case, where you're probably already unable to accept incoming connections.

It should be possible to disable this for private/testing deployments.