DaemonClient: new X-Hash-Difficulty HTTP header optimization

If the caller knows the difficulty of a PoW hash a given nonce
yields, it can tell the callee via the X-Hash-Difficulty, which
may allow the callee to skip some processing if the difficulty
does not meet some criterion.

In my case, a merge mining proxy can know it's pointless trying
to submit the nonce to a chain with higher difficulty when the
nonce only meets the difficulty for a lower difficulty chain.

src/base/net/stratum/DaemonClient.h

@@ -86,7 +86,7 @@ private:
     bool parseJob(const rapidjson::Value &params, int *code);
     bool parseResponse(int64_t id, const rapidjson::Value &result, const rapidjson::Value &error);
     int64_t getBlockTemplate();
-    int64_t rpcSend(const rapidjson::Document &doc);
+    int64_t rpcSend(const rapidjson::Document &doc, const std::map<std::string, std::string> &headers = {});
     void retry();
     void send(const char *path);
     void setState(SocketState state);