Add memory cache of Discovery Route query results.

I couldn't put this off any longer. Pandora Discover Doc is taking 20+ seconds.  With this change, after the first request the full Discover Doc will return in 500ms.  The REST route caches also shave off about 500ms from a request.

You can now request a limited Discover Document based on what ORMs you need.  That request looks like:
```
https://example.com/your-db-name-here/discovery/v1alpha1/apis/v1alpha1/rest?resources[]=Contact&resources[]=ClassCode&resources[]=Customer&resources[]=SalesOrder
```
This allows your client to get a Doc of whatever that client needs instead of the full Doc or a single resource Doc.