Ban IPs sending too many nodejoin messages

[dystophia]

This patch blocks IPs sending too many nodejoin messages inside the verifier code.

Keep in mind this may introduce new issues as its using extra memory for the message counters / banned list.

[EggPool]

@dystophia Could it be possible to have this helpful addition be optional, by the way of extra preference entries?

Main thing would be to not consume extra ram via nodejoinCounters if not activated (say nodejoinBanThreshold=0, by default)

This way, we could merge into the CE but have it activated and tuned via preferences only, not adding extra risk or ram usage if deactivated.

[EggPool]

Thanks for that work,
it has been superseded by #8 , built upon your code.

Closing this one since the upgrade version is now merged.