(🚧🚜 WIP 👷🚧 ) Split fabric-specific code into individual back-ends

[cchepelov]

(as being discussed on scalding-dev)

In the things I've left out for now (in addition to the list in the e-mail): scalding-hadoop-test (platform-specific tests on a real minicluster). Perhaps it'd make sense to have run / runHadoop (→runFabricLocal ?) / runOnMiniCluster in JobTest, and the relevant Mode implementation assortment?

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:white_check_mark: cchepelov
:x: Cyrille Chépélov (TP12)

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