NX-493: Metadata Service

Metadata service encapsulating:
* NX-493: metadata store based on OrientDB
* parsing (using low-memory Jackson Streaming API) of JSON documents
* producing streamed content (for downstream consumption)
* covering NX-498 hosted ("private registry") and NX-611 proxy ("registry proxy") and NX-612 group ("aggregated registry") use case for now
* NX-491 Use of GSON dropped