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

This makes the tests faster and avoids hypothesis complaints about assumptions
that fail too often.

Redeem the voucher once for each redemption group.
Track progress in the database.
Delay entering the "redeemed" state until all groups have succeeded.

This gives us more tokens than there are redemption groups, a requirement

They must use at least as many tokens as there are redemption groups

Previously the implementation allowed colliding passes if unblinded tokens
shared a prefix or suffix.

Allow voucher additions that are only distinct on the counter value.

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