NETWORKING: Enter SessionRequest

It is to be used in a Session, though it might be used separately. It
must implement keep-alive, but it does not yet.

You must not put it to ConnMan by yourself (instead, use start()) and
you must call close() after you've finished using this request.

You can either work with it in callback, or wait() and simply use its
methods (check it's success() and then, for example, use text()). Like
this:
```
Networking::SessionRequest *rq = new Networking::SessionRequest(url);
rq->startAndWait();
if (rq->success())
warning("HTTP GET: %s", rq->text());
rq->close();
```

backends/networking/curl/sessionrequest.cpp

@@ -0,0 +1,135 @@
+/* ScummVM - Graphic Adventure Engine
+ *
+ * ScummVM is the legal property of its developers, whose names
+ * are too numerous to list here. Please refer to the COPYRIGHT
+ * file distributed with this source distribution.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ */
+
+#define FORBIDDEN_SYMBOL_ALLOW_ALL
+
+#include <curl/curl.h>
+#include "backends/networking/curl/connectionmanager.h"
+#include "backends/networking/curl/networkreadstream.h"
+#include "backends/networking/curl/sessionrequest.h"
+#include "common/debug.h"
+#include "common/json.h"
+
+namespace Networking {
+
+SessionRequest::SessionRequest(Common::String url, DataCallback cb, ErrorCallback ecb):
+	CurlRequest(cb, ecb, url), _contentsStream(DisposeAfterUse::YES),
+	_buffer(new byte[CURL_SESSION_REQUEST_BUFFER_SIZE]), _text(nullptr),
+	_started(false), _complete(false), _success(false) {}
+
+SessionRequest::~SessionRequest() {
+	delete[] _buffer;
+}
+
+char *SessionRequest::getPreparedContents() {
+	//write one more byte in the end
+	byte zero[1] = {0};
+	_contentsStream.write(zero, 1);
+
+	//replace all "bad" bytes with '.' character
+	byte *result = _contentsStream.getData();
+	uint32 size = _contentsStream.size();
+
+	//make it zero-terminated string
+	result[size - 1] = '\0';
+
+	return (char *)result;
+}
+
+void SessionRequest::finishError(ErrorResponse error) {
+	_complete = true;
+	_success = false;
+	CurlRequest::finishError(error);
+}
+
+void SessionRequest::finishSuccess() {
+	_state = PAUSED;
+	_complete = true;
+	_success = true;
+
+	if (_callback)
+		(*_callback)(DataResponse(this, text()));
+}
+
+void SessionRequest::start() {
+	if (_started) {
+		warning("Can't start() SessionRequest as it is already started");
+	} else {
+		_started = true;
+		ConnMan.addRequest(this);
+	}
+}
+
+void SessionRequest::handle() {
+	if (!_stream) _stream = makeStream();
+
+	if (_stream) {
+		uint32 readBytes = _stream->read(_buffer, CURL_SESSION_REQUEST_BUFFER_SIZE);
+		if (readBytes != 0)
+			if (_contentsStream.write(_buffer, readBytes) != readBytes)
+				warning("SessionRequest: unable to write all the bytes into MemoryWriteStreamDynamic");
+
+		if (_stream->eos()) {			
+			finishSuccess();
+		}
+	}
+}
+
+void SessionRequest::restart() {
+	if (_stream)
+		delete _stream;
+	_stream = nullptr;
+	_contentsStream = Common::MemoryWriteStreamDynamic(DisposeAfterUse::YES);
+	_text = nullptr;
+	_complete = false;
+	_success = false;
+	//with no stream available next handle() will create another one
+}
+
+void SessionRequest::close() {
+	_state = FINISHED;
+}
+
+void SessionRequest::startAndWait() {
+	start();
+	wait();
+}
+
+bool SessionRequest::complete() {
+	return _complete;
+}
+
+bool SessionRequest::success() {
+	return _success;
+}
+
+char *SessionRequest::text() {
+	if (_text == nullptr)
+		_text = getPreparedContents();
+	return _text;
+}
+
+Common::JSONValue *SessionRequest::json() {
+	return Common::JSON::parse(text());
+}
+
+} // End of namespace Networking