[BEAM-3995] Build self-contained Nexmark jar

[kennknowles]

To run the Nexmark benchmarks, we will separate the build from the run step. The build step is owned by Gradle, while the run step is simply running a program. Nexmark is already designed this way.

WIP: the expected full set of dependencies is not contained in the shadow jar. I don't know why this is yet, but expect it has to do with how we manage the base setup. TBD.

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[kennknowles]

It is a big hammer; too big. We'll actually need to have a way to get the different runners on the classpath without bundling. But Gradle has nothing that I would care to use equivalent to mvn exec:java. Still poking around but the solutions I've found are not reasonable.

[kennknowles]

Ah, I think this is really not going to be workable. String munging to plumb cmdline args may still be what we need to do, in 2018.