The RMQ store will contain both the pending outgoing messages as well as the
unacknolwedged incoming message ids. Additionally, the device credentials
are stored there, with the security token encrypted using the local system
encryptor.
In order to support lightweight message passing across threads a MCSMessage
class is introduced.
BUG=284553
Review URL: https://codereview.chromium.org/56353002
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234413 0039d316-1c4b-4281-b951-d872f2087c98