Remove size limit in RPC client, keep it in server

The size limit makes a lot of sense for the server, as it never has to
accept very large data.

The client, however, can request arbitrary amounts of data with
`listtransactions` on a large wallet.

Fixes #4604.

src/bitcoin-cli.cpp

@@ -126,7 +126,7 @@ Object CallRPC(const string& strMethod, const Array& params)
     // Receive HTTP reply message headers and body
     map<string, string> mapHeaders;
     string strReply;
-    ReadHTTPMessage(stream, mapHeaders, strReply, nProto);
+    ReadHTTPMessage(stream, mapHeaders, strReply, nProto, std::numeric_limits<size_t>::max());
 
     if (nStatus == HTTP_UNAUTHORIZED)
         throw runtime_error("incorrect rpcuser or rpcpassword (authorization failed)");

src/rpcprotocol.cpp

@@ -201,14 +201,14 @@ int ReadHTTPHeaders(std::basic_istream<char>& stream, map<string, string>& mapHe
 
 int ReadHTTPMessage(std::basic_istream<char>& stream, map<string,
                     string>& mapHeadersRet, string& strMessageRet,
-                    int nProto)
+                    int nProto, size_t max_size)
 {
     mapHeadersRet.clear();
     strMessageRet = "";
 
     // Read header
     int nLen = ReadHTTPHeaders(stream, mapHeadersRet);
-    if (nLen < 0 || nLen > (int)MAX_SIZE)
+    if (nLen < 0 || (size_t)nLen > max_size)
         return HTTP_INTERNAL_SERVER_ERROR;
 
     // Read message

src/rpcprotocol.h

@@ -153,7 +153,7 @@ bool ReadHTTPRequestLine(std::basic_istream<char>& stream, int &proto,
 int ReadHTTPStatus(std::basic_istream<char>& stream, int &proto);
 int ReadHTTPHeaders(std::basic_istream<char>& stream, std::map<std::string, std::string>& mapHeadersRet);
 int ReadHTTPMessage(std::basic_istream<char>& stream, std::map<std::string, std::string>& mapHeadersRet,
-                    std::string& strMessageRet, int nProto);
+                    std::string& strMessageRet, int nProto, size_t max_size);
 std::string JSONRPCRequest(const std::string& strMethod, const json_spirit::Array& params, const json_spirit::Value& id);
 json_spirit::Object JSONRPCReplyObj(const json_spirit::Value& result, const json_spirit::Value& error, const json_spirit::Value& id);
 std::string JSONRPCReply(const json_spirit::Value& result, const json_spirit::Value& error, const json_spirit::Value& id);

src/rpcserver.cpp

@@ -891,7 +891,7 @@ void ServiceConnection(AcceptedConnection *conn)
             break;
 
         // Read HTTP message headers and body
-        ReadHTTPMessage(conn->stream(), mapHeaders, strRequest, nProto);
+        ReadHTTPMessage(conn->stream(), mapHeaders, strRequest, nProto, MAX_SIZE);
 
         // HTTP Keep-Alive is false; close connection immediately
         if (mapHeaders["connection"] == "close")