MINOR: htx: Store the head position instead of the wrap one

MINOR: htx: Store the head position instead of the wrap one

The head of an HTX message is heavily used whereas the wrap position is only
used when a block is added or removed. So it is more logical to store the head
position in the HTX message instead of the wrap one. The wrap position can be
easily deduced. To get it, the new function htx_get_wrap() may be used.