[cppmyth] test: show under buffer
[cppmyth] Fix buffering

- Backend could crash when opening a 0B file:
 Here we wait the filling before opening transfer. Finally the file will be opened on switch.
- Read buffered data without waiting request response:
The playback has to own tcp sockets to select control and data at same time. So a new 'read' method is now available only from the playback which own the control and the transfer: TransferRequestBlock().