ISIS-597: session now open during service initialization.

Also changes to example app isis.properties to ensure that RegisterEntities
is called first as a service (make sure that JDO metamodel is initialized)
ISIS-611: call RegisterEntities from DataNucleusPersistenceMechanismInstaller

No longer necessary to register as a service (the key is still needed, indeed is now mandatory).
ISIS-611: removing RegisterEntities as a service...

... from the example app *IntegTestBuilder classes (since no longer reqd)