Make JsonTree and JsonSchemaTree into interfaces

A slightly better design. And I can mock them, now...

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>
Start and implement the new message API

Start with the base implementation of ProcessingMessage, and a failing test to
go with it.

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>