[mythtv-cmyth] Fix different buffer size depending of  backend connection type.

- For chain, file and path, buffer is only used to receive backend response of query ANN FileTransfer. So 64 bytes is large enough. This value is defined by RCV_BUF_DATA_SIZE.
- For artworks downloader, the buffer size is fixed to 32000. This value is defined by RCV_BUF_IMAGE_SIZE.
- For control, buffer is fixed to 32000. This value is defined by RCV_BUF_CONTROL_SIZE.