MEDIUM: spoe: Make the SPOE applet use its own buffers

MEDIUM: spoe: Make the SPOE applet use its own buffers

The SPOE applet is rewritten to use its own buffers. It is not a huge change
because, once started, the only responsibility of the SPOE applet is to
transfer the ACK frame to the SPOE filter. So it means it does not send any
data to the opposite endpoint, the NOTIFY frame was already transferred
during the applet creation. And it does only receive one full frame. Once
received, it can exit.

The related issue is #2502.