Note: this depends on D479!
This is a port of Bitcoin Classic's strategy to ignore non-Cash peer
ADDR messages if already on the Cash chain.
Since deadalnix seemed to approve of this for Bitcoin ABC based on slack
conversations in #dev, I went ahead and implemented it.
Note that Classic has a global "Chain state is CASH" type of variable for
quick and cheap determinations if we're in the bitcoin cash chain state.
UPDATE: I made this code use a a global flag to determine if on CASH chain (requires D479)