Move the secure variables quota enforcement calls into the state store to ensure
quota checks are atomic with quota updates (in the same transaction).
Switch to a machine-size int instead of a uint64 for quota tracking. The
ENT-side quota spec is described as int, and negative values have a meaning as
"not permitted at all". Using the same type for tracking will make it easier to
the math around checks, and uint64 is infeasibly large anyways.
Add secure vars to quota HTTP API and CLI outputs and API docs.
