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

Merge pull request #126 from PrivateStorageio/125.backup-order-equals-spend-order-equals-insert-order

Make backup order equal spend order equal insert order

We have our new test that verifies ordering

Cause extraction to fail if it cannot supply all requested tokens

Bust the cache key