Get messages by id

This patch modifies the "get" method in the storage drivers' message
controller to allow querying a set of messages by id.

In the next patch, the WSGI transport driver will be similarly
modified to allow querying for multiple messages by ID, through
the API.

Change-Id: I3bd183438a3eaf7af6ca52a19bb804c47ac1f245
Implements: blueprint v1-api