MINOR: mux-h1: Add embryonic and attached states on the H1 connection

MINOR: mux-h1: Add embryonic and attached states on the H1 connection

Be prepared to have a H1 connection in one of the following states :

 * A H1 connection waiting for a new message with no H1 stream.
   H1C_F_CS_IDLE flag is set.

 * A H1 connection processing a new message with a H1 stream but no
   conn-stream attached. H1C_F_CS_EMBRYONIC flag is set

 * A H1 connection with a H1 stream and a conn-stream attached.
   H1C_F_CS_ATTACHED flag is set.

 * A H1 connection with no H1 stream, waiting to be released. No flag is set.

These flags are mutually exclusives. When none is set, it means the
connection will be released ASAP, just remaining outgoing data must be sent
before. For now, the second state (H1C_F_CS_EMBRYONIC) is transient.