Report minfeefilter value in getpeerinfo rpc

Lowering the minimum relay fee is only useful when many nodes in the
p2p network also lower the fee, so to make it easier to understand
progress on that front, this includes the value of the minfeefilter in
getpeerinfo, so you at least have visibility to what fees your neighbours
are currently accepting.

src/net.h

@@ -558,6 +558,7 @@ public:
     double dPingTime;
     double dPingWait;
     double dMinPing;
+    CAmount minFeeFilter;
     // Our address, as reported by the peer
     std::string addrLocal;
     // Address of this peer