MINOR: mux-h1: Xfer as much payload data as possible during output processing

MINOR: mux-h1: Xfer as much payload data as possible during output processing

When an outgoing HTX message is formatted to a raw message, DATA blocks may be
splitted to not tranfser more data than expected. But if the buffer is almost
full, the formatting is interrupted, leaving some unused free space in the
buffer, because data are too large to be copied in one time.

Now, we transfer as much data as possible. When the message is chunked, we also
count the size used to encode the data.