approach which assumes a dedicated node for observers: states that are
reported to the node will appear in the database and update feeds as
normal. Apps that expect all updates to be relevant to themselves may
need adjusting if they run on an observer node too, but this is likely
to be rare.
* Fixed AbstractNode to load custom notary services properly.
Added a custom notary sample.
* Prevent multiple custom notaries from being loaded
* Throw if more than once custom notary service is loaded
