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.

We're going to start looking at it.  It's not reasonable for its value to be None.

This exercises whitespace and no-whitespace cases.

... on purpose so devs know what to expect from users like me.

It is easy to erroneously add extra white space to a text file. For example when writing with an editor like `nano` or when using `echo` without the `-n` argument.

Handing that white space over to python-challenge-bypass-ristretto's `decode_base64()` method will make it fail in a rather opaque way, see https://github.com/LeastAuthority/python-challenge-bypass-ristretto/issues/37.

This eliminates white space including newlines around the key expected to be in that file.

The redeemer will submit this many tokens with its redemption request now.  It
needs to agree with whatever the issuer wants to do, of course.

It builds _Configs instead of functions that return _Configs.

Also add a test that actually serializes it

Otherwise idle tahoe processes consume enormous amounts of CPU time.

Fixes #177