We can just have Hypothesis give us a POSIX timestamp and then twiddle our
clock with that value appropriately.  Not much less code but a lot less
explanation.

This is better than stuffing it in later.

Tahoe-LAFS 1.16 changes the way StorageServer determines the current time.
This is great because it gives us a nice place to plug in our fake time.  We
still have to support <1.16 which leaves some not-so-nice stuff around.

If RIStorageServer doesn't define it, don't try to wrap it.

Previously Hypothesis spent a long while putting storage indexes on the
storage servers and creating a list of nodes with non-overlapping storage
indexes and then asserting that, out of no relevant shares, none had expired
leases.

Now all storage servers in the test will have all of the relevant shares and
so all should be visited and updated.

It's better if we don't have to scrape the public key out of the config text,
anyway.

It is observed for each redemption group and it could vary for groups
belonging to a single voucher.

Take care not to spend them, though.

It doesn't make the test much faster ... but maybe a little?  It does slightly
improve failures by producing fewer giant base64 strings.

This will make it easier to add the new allowed-public-keys item which must be
present regardless of chosen redeemer.