Previously an active lease on *any* share would cause us to disable lease
renewal entirely - including for newly written shares.

Now we will add a lease to any changed share that needs one, either because it
is growing or because it has no unexpired lease.

This means the test suite can actually exercise logic that depends on
different lease expiration times for different shares in one bucket.

slot_testv_and_readv_and_writev will manage this state for now.
later we'll figure out how to just disable it entirely.

This requires a newer Tahoe to provide the `set_implicit_bucket_lease_renewal`
function.  Then update our wrapper to use it.  Also add a test for the
behavior.

There is an effect on mutables as well since Tahoe changed how you turn off
implicit lease renewal for them as well.  Just update to use the newer API to
keep things working as they were.

This removes the need to call `registerAdapter` on
`ZKAPAuthorizerStorageServer`. It looks like that was needed because the
interfaces for it were defined by `implementer_only` which explicitly ignored
interfaces defined by parent classes. Looking at the history, I assume the use
of `implementer_only` was to avoid implementing `RIStorageServer` via
`proxyForInterface`. Given that the later is no longer used, `implementer_only`
is no longer necessary.

Meant this to be in PR#152 but somehow forgot to even commit it...

Also centralize the exception-raising in one place

And update tests to take advantage of free equality we get

Compute it ourselves so it is correct for large files

This removes almost all uses of the global value.  All that remains is its use
as a default in case no configuration is given.

This helps the client a lot with lease operations