API v1.1: adding id field to messages

Now message's dict in responses contains 'id' field. Affected endpoints
are:

get message by id: GET /v1.1/queues/{queue}/messages/{message_id}
list messages: GET /v1.1/queues/{queue}/messages?{ids}
pop messages: DELETE /v1.1/queues/{queue}/messages?pop=

Change-Id: I4ca5a5ef22abe05b3d7bc39ebe7d1fc742e21eb4
Partially-implements: blueprint api-v1.1-response-document-changes
Merge "API v1.1: adding id field to messages"