Do not use Strings for internal messages

Do not use Strings for internal messages

Using a String for the message identity has the effect of the equality
running on Strings, which has the effect that message is not really
isolated, but can be sent by anyone who knows the message content.

Instantiate simple Objects instead, which force a proper identity
equality check. Retain debuggability by having these instances override
toString().

Change-Id: Ia581e0a1e023ace10b5dbb81a44092ca04a4ff8f
Signed-off-by: Robert Varga <rovarga@cisco.com>