Implement "feefilter" P2P message.

The "feefilter" p2p message is used to inform other nodes of your mempool min fee which is the feerate that any new transaction must meet to be accepted to your mempool.  This will allow them to filter invs to you according to this feerate.

src/policy/fees.h

@@ -286,4 +286,17 @@ private:
     CFeeRate feeLikely, feeUnlikely;
     double priLikely, priUnlikely;
 };
+
+class FeeFilterRounder
+{
+public:
+    /** Create new FeeFilterRounder */
+    FeeFilterRounder(const CFeeRate& minIncrementalFee);
+
+    /** Quantize a minimum fee for privacy purpose before broadcast **/
+    CAmount round(CAmount currentMinFee);
+
+private:
+    std::set<double> feeset;
+};
 #endif /*BITCOIN_POLICYESTIMATOR_H */