BUG/MEDIUM: mux-spop: Remove frame parsing states from the SPOP connection state

BUG/MEDIUM: mux-spop: Remove frame parsing states from the SPOP connection state

SPOP_CS_FRAME_H and SPOP_CS_FRAME_P states, that were used to handle frame
parsing, were removed. The demux process now relies on the demux stream ID
to know if it is waiting for the frame header or the frame
payload. Concretly, when the demux stream ID is not set (dsi == -1), the
demuxer is waiting for the next frame header. Otherwise (dsi >= 0), it is
waiting for the frame payload. It is especially important to be able to
properly handle DISCONNECT frames sent by the agents.

SPOP_CS_RUNNING state is introduced to know the hello handshake was finished
and the SPOP connection is able to open SPOP streams and exchange NOTIFY/ACK
frames with the agents.

It depends on the following fixes:

  * MINOR: mux-spop: Don't set SPOP connection state to FRAME_H after ACK parsing
  * BUG/MINOR: mux-spop: Make the demux stream ID a signed integer

This change will be mandatory for the next fix. It must be backported to 3.1
with the commits above.