go sdk: improve caching mechanisms for keys and attribute definitions

[dmihalcik-virtru]

• These should be customizeable from the application
• These should all be consistent - that is, we should try to make sure during a single application level operation, it will get a version of the KAS and Policy information that represents a consistent view of the data. Eg it won't operate on versions 1 and 2 of an attribute definition (e.g. with a hierarchy rule with a missing value, a key access grant with a different set of allowed kases, or using two different keys for the same KAS and algorithm type)