Currently builds are failing to upload coverage reports because:

```
==> Uploading
    .url https://codecov.io
    .query service=circleci&package=py2.0.15&job=1057.0&yaml=codecov.yml&token=<secret>&build=1057.0&branch=154.save-and-respond.1&commit=714379bf&slug=PrivateStorageio%2FZKAPAuthorizer
    Pinging Codecov...
    Uploading to S3...
Error: 400 Client Error: Bad Request for url: https://storage.googleapis.com/codecov/v4/raw/2020-05-26/3F10B6B6FEA878CCC9F58EAF87855DA8/714379bfb318ec39566729b2fecce6339ff55daf/17397def-dfec-47bb-b20f-879e223b0e90.txt?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=GOOG1EQX6OZVJGHKK3633AAFGLBUCOOATRACRQRQF6HMSMLYUP6EAD6XSWAAY%2F20200526%2FUS%2Fs3%2Faws4_request&X-Amz-Date=20200526T132703Z&X-Amz-Expires=10&X-Amz-SignedHeaders=host&X-Amz-Signature=86369f603a1c27782899d7df6206b35a4564c70fe4efaea3cb72dd03cfb80b36

Tip: See all example repositories: https://github.com/codecov?query=example
Support channels:
  Email:   hello@codecov.io
  IRC:     #codecov
  Gitter:  https://gitter.im/codecov/support
  Twitter: @codecov
```

Retry with new passes if an operation fails because the server rejected some passes

If all passes were good but there simply were not enough of them, don't retry.

Replace the tricky _rref property with a decorator.  This guarantees _get_rref
is evaluated for each function call before we do anything with pass generation.

Same logic should apply to the rest of the pass-spending methods.

Then, logic can be extended to cover retry for other failures and putting
passes back in the database for non-spending failures and converged uploads.

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

Make the storage server report which passes it is rejecting and give a reason for it

Also centralize the exception-raising in one place

And update tests to take advantage of free equality we get

Consistent voucher-redemption ZKAP count

The 2nd get_tokens is never used so the line it is defined on is considered
partially covered.  Replace the lambda with None and explain what's going on.

Have PaymentController iterate a voucher's redemption counters

This condition can never succeed.

If the voucher is being seen for the first time at this point then it should
start redemption and the check will not pass.

If the voucher has been seen before and was in an error state to begin with
then the check higher up, outside of the loop, prevents execution from ever
entering the loop.

If the voucher has been seen before and just now entered an error state then
the `if not succeeded` check below, on the previous iteration, would stop the
loop and prevent this condition from ever being checked.

Pin Tahoe-LAFS to 1.14.0

Why is pip installing "Twisted[tls]>16.4" when Tahoe-LAFS asks for
"Twisted[tls,conch] >= 18.4.0"?

Compute >= 2 GiB share size correctly

Fix test coverage collection and reporting from the Nix CI job

Also document some of the other decisions in the .coveragerc file

The paths are even weirder inside the build environment.

We're abusing combine to rewrite paths in the coverage data.  combine isn't
intended to be used with only one coverage data file named ".coverage".  So
enable parallel mode so we have only one coverage data file with the naming
scheme "combine" expects.

Maybe this gets us working path rewriting on Windows,

codecov docs say that xml is supported and explicitly call out Python
".coverage" format as unsupported.

https://docs.codecov.io/docs/supported-report-formats