Making user functions serializable in high-level APIs

In order to be able to fully serialize the user DAG in high-level APIs, we need to make sure that the user-supplied functions are serializable as well. This will enable:

1. Deserialize and instantiation of per-task user functions become completely managed by the system. No need to invoke StreamApplication.init() in each task
2. Fully serialized user DAG can be distributed across the network to multiple containers