Considering `new_length` in `slot_testv_and_readv_and_writev` for storage usage is error prone and/or complicated

Our storage server tries to calculate the cost of storage using either the real amount of data stored or the virtual maximum size of shares deriving from the new_length field in tw_vectors (which can either extend or truncate a share without actually writing any data).

It seems reasonable to consider the virtual size of a share when calculating costs since by accepting such an operation we're strongly suggesting we will eventually accept the real data for the share.

However, actually correctly calculating storage usage accounting for new_length proves complicated and quite difficult without violating a bunch of abstraction boundaries in Tahoe-LAFS (for example, the virtual size of a share isn't exposed via any public-looking Python API).