Add support for read-only managed objects

Support the notion of a read-only managed object. This could be transparent to the application by
automatically determining that an object is immutable, or exposed to the developer statically through
annotation or interface(s), or dynamically via APIs. A dynamic approach would provide an straight forward mechanism for applications to globally "lock" objects. Any method would provide the ability for Darkstar to optimize accesses to such objects, potentially reducing data store and network activity.