Add a serial collection-based execution mode

Summary of mailing list thread that the issue is based upon

Since Flink is a layered system, programs written against the APIs can be executed in a variety of ways. In this case, we just run the functions single-threaded directly on the Java collections, instead of firing up a memory management, IPC, parallel workers, data movement, etc. That gives programs a minimal execution footprint (like in the Java8 streams API) for small data. The idea is to enable users to use the same program in all sorts of different contexts.

The collection execution should sit below the common API, so both Java and Scala API can use it.