get_transfer_buffer is poor design

[JayFoxRox]

get_transfer_buffer was added during review of #17 to avoid leaking memory.

However, this design is quite poor:

• The buffer is never free'd and only grows. This means memory is wasted which might be necessary for other tasks while not transferring data. This would be especially bad when running in the background of a game.
• Only a single buffer can be allocated and changing it's size makes it loose the content.
• It's not thread safe.

Let me be clear: What we have works absolutely fine. But in the future we might want multiple buffers per transfer or we need to reduce the nxdk-rdt memory footprint. In those cases, this issue will become important.