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/protocol.cpp

@@ -34,6 +34,7 @@ const char *FILTERADD="filteradd";
 const char *FILTERCLEAR="filterclear";
 const char *REJECT="reject";
 const char *SENDHEADERS="sendheaders";
+const char *FEEFILTER="feefilter";
 };
 
 static const char* ppszTypeName[] =
@@ -68,7 +69,8 @@ const static std::string allNetMessageTypes[] = {
     NetMsgType::FILTERADD,
     NetMsgType::FILTERCLEAR,
     NetMsgType::REJECT,
-    NetMsgType::SENDHEADERS
+    NetMsgType::SENDHEADERS,
+    NetMsgType::FEEFILTER
 };
 const static std::vector<std::string> allNetMessageTypesVec(allNetMessageTypes, allNetMessageTypes+ARRAYLEN(allNetMessageTypes));