Retrieval metrics GET endpoint

[jcace]

Depends on #17

Re-work the retrieval metrics GET endpoints and wire them up to the new schema.

The new endpoint should take params:

• timeStart : Timestamp (epoch) to start query from
• timeEnd : Timestamp (epoch) to end query
• sp <string | optional>: Storage provider ID to limit the query to (return all if undefined)

The endpoint should return an array of Retrieval Event objects