MAJOR: spoe: Add support of payload fragmentation in NOTIFY frames

MAJOR: spoe: Add support of payload fragmentation in NOTIFY frames

Now, agents can announce the support for the "fragmentation" capability during
the HELLO handshake. HAProxy will never announce it because fragmented frame
decoding is not implemented yet. But it can send such kind of frames. So, if an
agent supports this capability, payloads exceeding the frame size will be
split. A fragemented payload consists of several frames with the FIN bit clear
and terminated by a single frame with the FIN bit set. All these frames must
share the same STREAM-ID and FRAME-ID.

Note that an unfragemnted payload consists of a single frame with the FIN bit
set. And HELLO and DISCONNECT frames cannot be fragmented. This means that only
NOTIFY frames can transport fragmented payload for now.