Refactor ObservationManagerFactory

The current o.a.j.core.observation.ObservationManagerFactory class has two main responsibilities:

1) Create new ObservationManagerImpl instances as an observation manager factory
2) Manage event consumers and dispatch events within a workspace

These two responsibilities are quite unrelated and the factory responsibility essentially boils down to the following method that is only ever invoked within WorkspaceImpl.getObservationManager():

public ObservationManagerImpl createObservationManager(SessionImpl session, ItemManager itemMgr)

{ return new ObservationManagerImpl(this, session, itemMgr); }

To simplify the design I'd inline this method and rename ObservationManagerFactory to ObservationDispatcher to better reflect the one remaining responsibility.