Major Description
The MessageHandler/MessageService supports requests for getting messages by id, but JsonDbOpensocialService does not currently implement filtering by id.
The attached patch implements that feature.
Please note: in the current Opensocial 2.5.1 spec (http://opensocial.github.io/spec/2.5.1/Social-API-Server.xml#rfc.section.2.7.1) it seems that only a single message id is supported, and depending on how you read the section the container should return the message itself, not a singleton collection of messages in that case.