Simplify the code that reads from the socket line by line

Since sockets are buffered `QIODevices` we can use `canReadLine()` to check
if we have a full line, instead of using a custom `SocketLineReader` class
(and the copy-pasted `DeviceLineReader` in the Bluetooth backend). 

We can also loop through all the lines instead of queuing calls to `dataReceived`.

And we don't need transactions.