Replace the protocol buffer use in the runtime API with POJOs

We currently use Protocol Buffers between REEF and any particular runtime implementation (reef-runtime-local,reef-runtime-yarn, reef-runtime-mesos and reef-runtime-hdinsight). We choose this approach based on the assumption that there might be a process boundary between the runtime implementation and the rest of the Driver.

However, that assumption hasn't been true for any of the runtimes we have so far. Hence, the use of Protocol Buffers for this API complicates matters for no gain. It would therefore be a good idea to drop it and replace the API with a standard, Plain Old Java Object (POJO) based approach.

This refactoring would be an excellent, if somewhat painful, way to understand the guts of REEF for new committers and contributors.