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.
This code is less than ideal because to make the final decision it has
to hold cs_main for a tiny amount of time to check chainActive.Tip()
in the call to IsUAHFEnabledForCurrentBlock().
We should think about having a global flag for fActiveChainIsUAHF or
something along those lines.. but that can go in a later diff and I can
then modify this code to use it.