Session storage prototype. #26
Session storage. closes #26
Some fixes by Roman's review.
Minimize patch by Roman's review. #26
box.session.storage: minor changes
gh-26: minor optimization of session destructor

The patch should use struct session, which is non-existent
in 1.5, so at least make session_storage_destroy a bit faster.
Merge branch '26-session-storage' into stable