Remove Instance<ConversationContext> from AbstractConversationContext and ConversationImpl

Since all the conversations are accessing the same instance of Instance<ConversationContext>, we effectively have a global lock (InstanceImpl extends AbstractFacade, which uses CreationalContextImpl, which synchronizes on field dependentInstances).

We can get the active ConversationContext through beanManagerImpl.getContext() instead of through Instance<ConversationContext>