Encapsulate top-level lists in responses in object

All top-level JSON arrays in response bodies are now encapsulated within
an object. Endpoints changed:
messages_get_many: GET /v1.1/queues/{queue}/messages?ids=
claim_create: POST /v1.1/queues/{queue}/claims

Partially-implements: blueprint api-v1.1-response-document-changes
Change-Id: I81e64decc928be665b8b4f4d0e238fd701794bc9
Merge "Encapsulate top-level lists in responses in object"