MINOR: htx: Add the start-line offset for the HTX message in the HTX structure

MINOR: htx: Add the start-line offset for the HTX message in the HTX structure

If there is no start-line, this offset is set to -1. Otherwise, it is the
relative address where the start-line is stored in the data block. When the
start-line is added, replaced or removed, this offset is updated accordingly. On
remove, if the start-line is no set and if the next block is a start-line, the
offset is updated. Finally, when an HTX structure is defragmented, the offset is
also updated accordingly.