Servers require and clients supply passes correctly for renew_lease.

Check the number of passes in add_lease.

This helps the client a lot with lease operations

The native st_size-based approach to size estimation is broken because mutable
and immutable layouts have a different amount of overhead.  So start peeking
inside the share files.

Also, new_length must be respected now because when it is used the part of the
share file we peek at changes.  Also, new_length can *only* truncate so stop
treating it as if it can extend.

Do it another way.

Disregard `new_length` for pass calculations in `slot_testv_and_readv_and_writev`.

and without new_length

Instead of assuming all shares are the same size here, pass in all the sizes.
The protocol doesn't allow immutable shares in a storage index to have
differences in size but it does allow mutable shares in a slot to differ this
way.

Supply and enforce a correct ZKAP count for mutable writes

Deny writes to shares without an active lease.

Also make it use slot_testv_and_readv_and_writev instead of slot_readv

The behavior should be the same and slot_readv is simpler and less
interesting.