Refactoring InstantiatorFactory to allow per-deployment configuration

InstantiatorFactory is currently using static fields for storing the enabled/disabled status for unsafe instantiators, as well as the instantiator that should be used. In a shared library scenario, the instantiator factory will be enabled if at least one of the deployed applications contains the META-INF/org.jboss.weld.enableUnsafeProxies resource.