Use event loop for data transfer

[IlyaBizyaev]

HTTPFileServer is not as Qt friendly as we want it to be: it uses a loop that blocks signal processing and makes big files cause a crash on Windows.

What is not a solution: flush(), waitForBytesWritten(), QCoreApplication::processEvents()
Solution: signals and slots, as most of QTcpSockets' functionality is implemented as those.

See http://doc.qt.io/qt-5/qabstractsocket.html#flush and http://doc.qt.io/qt-5/qabstractsocket.html#waitForBytesWritten for reference.

[IlyaBizyaev]

Another problem our loop introduces is that multiple renderers cannot be used in parallel.

[IlyaBizyaev]

Also, seek is not possible if the renderer prefers parallel requests (e.g. Kodi on Windows).