MINOR: spoe: Use the min of all known max_frame_size to encode messages

MINOR: spoe: Use the min of all known max_frame_size to encode messages

The max_frame_size value is negociated between HAProxy and SPOE agents during
the HELLO handshake. It is a per-connection value. Different SPOE agents can
choose to use different max_frame_size values. So, now, we keep the minimum of
all known max_frame_size. This minimum is updated when a new connection to a
SPOE agent is opened and when a connection is closed. We use this value as a
limit to encode messages in NOTIFY frames.