Adding max_storage_mb to Service Instance

* Now that we support multiple plans, storing the max_storage_mb quota
* per service instance will make it easier to enforce quota

[#67082390]
Adding db_name to Service Instance schema

* This will help make our enforcement query easier to understand and
* perform better.

[#67082390]
Implemented Quota Enforcer for multiple plans

[#67082390]
Fixing broken specs after refactoring Quota Enforcement

[#67082390]