addrman: Do not propagate obviously poor addresses onto the network

This is a first pass at filtering bad addresses.

**ETA**:  Although this might not be a perfect solution, we need to stop the practice of returning long dead garbage addresses to other peers.  This addresses one of the longstanding Things That Bug Me About Bitcoin.
You've changed the type of nNodes to unsigned already. Maybe use an unsigned int or size_t counter here as well instead of an int - no need for the (int) cast in that case, which could theoretically be a truncation.