MINOR: mux-h2: Use flags of HTX blocks to detect end of message during sending

MINOR: mux-h2: Use flags of HTX blocks to detect end of message during sending

When a message is sent, the end of the message is now detected by testing
flags of each HTX blocs. Otherwise, most of the logic remains unchanged. The
only difference is the detection of the end of the message when headers are
sent (back or front). Now, it is be possible to detect the EOM when the
stream is already closed and H2_SF_MORE_HTX_DATA may be removed while it was
untouched before. But it is not a big deal because this flag is only tested
when a shutdown is performed on the stream. So removing it for an already
closed stream is a no-op.