MINOR: mux-h2: force reads to be HTX-aligned in HTX mode

MINOR: mux-h2: force reads to be HTX-aligned in HTX mode

H2 has a 9-byte frame header, and HTX has a 40-byte frame header.
By artificially advancing the Rx header and limiting the amount of
bytes read to protect the end of the buffer, we can make the data
payload perfectly aligned with HTX blocks and optimize the copy.