Carry the accepted quote into each request, persist it, and reject
invoices above that limit. Preserve compatibility for requests that
omit the cap while distinguishing an explicit zero.
Provide a wrapped store type, exposing ExecTx method with a subset
interface in the callback argument. BaseDB interfaces in instantout,
reservation and sweepbatcher use ExecTx with their subset Querier
instead of whole sqlc.Querier (*sqlc.Queries).
This is needed to make the packages more reusable, so they don't
depend on methods of *sqlc.Queries they don't use.
A Querier holds the methods of sqlc.Querier interface relevant for a package.
BaseDB has Querier + ExecTx method.
This change is needed to simplify further rework of ExecTx.
