MAJOR: h3: Implement zero-copy support to send DATA frame

MAJOR: h3: Implement zero-copy support to send DATA frame

When possible, we try send DATA frame without copying data. To do so, we
swap the input buffer with QCS tx buffer. It is only possible iff:

 * There is only one HTX block of data at the beginning of the message
 * Amount of data to send is equal to the size of the HTX data block
 * The QCS tx buffer is empty

In this case, both buffers are swapped. The frame metadata are written at
the begining of the buffer, before data and where the HTX structure is
stored.