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.

this seems likely to be wrong.
it's probably fine for slot creation but for later writes to a slot it will
probably underestimate the passes required.
we probably have to look at the "data length" of any existing share

Immutable shares are limited to what you can pay for with 10 passes right
now.  This certainly needs to be lifted but until it is we don't want spurious
test failures telling us about the limit.

Mutable shares are limited to what you can pay for with 1 pass right now
because the client doesn't know how to compute the right number of passes and
always spends 1 pass.

Integer division lets this number be zero sometimes.

If enough valid passes are supplied for the amount of storage requested then
allow the operation to succeed.  Otherwise, fail with an error.

We do so by introducing a limitation that the client must be configured with
an issuer and it must match the issuer announced by any storage server the
client is to talk to.

The client will decline to speak to any storage server without a matching
issuer.