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

Something fails with "No usable data files" on CI, not sure what