Disable the Eliot test suite in the Nix package build

Emit an Eliot message when spending happens at the lowest level

A ZKAP is now worth 1 MiB instead of 128 KiB

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

Call redeemWithCounter with a Voucher object instead of voucher text

Add counter to database and Python representation

A new voucher has not been redeemed any times.

Replace IRedeemer.redeem with a counter-accepting API and update callers

Add an interface for inserting tokens