journal: Use 32-bit entry item object offsets in compact mode

journal: Use 32-bit entry item object offsets in compact mode

To do this, we move EntryItem out of journal-def.h and turn it into
a host only struct in native endian mode so we can still use it to
ship the necessary info around.

Aside from that, the changes are pretty simple, we introduce some
extra functions to access the right field depending on the mode and
convert all the other code to use those functions instead of
accessing the raw fields.

We also drop the unused entry item hash field in compact mode. We
already stopped doing anything with this field a while ago, now we
actually drop it from the format in compact mode.