Make rate-limiting and quota rate-limiting plugins work when DB is down

[subnetmarco]

Introduces a new option to establish if the rate-limiting and response rate-limiting plugin should work even if the DB is down.

This means that if provider decides to enable this feature, and the DB is down, requests will go through anyways. If the feature is disabled instead (default behavior), then the user will see 500 errors like he used to.