Reintroduce Entry::getItem optimization in next api/abi release.

[mgautierfr]

PR #768 remove interesting optimization but it was necessary as the change introduced a ABI break.
We should reintroduce it in the next release of libzim with ABI break

[kelson42]

@mgautierfr I'm not sure there is anything to do here as you have made 8.1.1 in a branch?!

[mgautierfr]

This is just a reminder to not forget to reintroduce the change.
We could reintroduce it right now, it it would force us to make a version 9.
As this change is just a really small optimization, I prefer to postpone it and make it when other change forces us to have a major version change.

[kelson42]

@mgautierfr We should really do that right now IMO