Implement "feefilter" P2P message.

src/protocol.h

@@ -211,7 +211,12 @@ extern const char *REJECT;
  * @see https://bitcoin.org/en/developer-reference#sendheaders
  */
 extern const char *SENDHEADERS;
-
+/**
+ * The feefilter message tells the receiving peer not to inv us any txs
+ * which do not meet the specified min fee rate.
+ * @since protocol version 70013 as described by BIP133
+ */
+extern const char *FEEFILTER;
 };
 
 /* Get a vector of all valid message types (see above) */