Consider a token caching facility

[jessepeterson]

Apple has been adding tokens to DEP such as the MAID JWT and Beta OS tokens. Consider adding a token caching facility to NanoDEP. Some ideas/considerations:

• Light-weight, simple, perhaps just a KV (dict/map/hash) store
• Flexible cache invalidation depending on token-type
• Arbitrary enough for unknown future tokens
• Perhaps just a raw DEP fetch cache/middleware? I.e. potential to work for both depserver proxy and godep.
• Further consideration on token keying: e.g. is it per-account (DEP name), per-device, other?
• Invalidation on OAuth and/or token PKI changes