storage: introduce v2 immutable shares, with 8-byte offsets fields, to remove two...

storage: introduce v2 immutable shares, with 8-byte offsets fields, to remove two of the three size limitations in #346. This code handles v2 shares but does not generate them. We'll make a release with this v2-tolerance, wait a while, then make a second release that actually generates v2 shares, to avoid compatibility problems.