Re-enable cached JSON-LD document loader for zcaps

[llorllale]

There is a bug in the framework when using the CachingDocumentLoader from json-gold that causes the server to panic and die.

It turns out the framework is automatically creating a CachingDocumentLoader and always adding the context we need (https://w3id.org/security/v1, https://w3id.org/security/v2). It's not safe to use the same CachingDocumentLoader instance across all verifications in different goroutines.