[BEAM-1087] Fix inconsistent pickle from dill when dumping the session

[sb2nov]

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This PR tries to fix GoogleCloudPlatform/DataflowPythonSDK#31 with adding a reduce hook to the Step and Job classes. We also need to modify the dump session so that the pickling is consistent due to uqfoundation/dill#195.

R: @robertwb PTAL

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[sb2nov]

R: @robertwb PTAL

[robertwb]

Ideally we shouldn't be pickling any live objects from apiclient.py... can this be avoided?

[sb2nov]

There is a dangerous looking solution in uqfoundation/dill#66 which could be useful in ignoring the objects in apiclient but I think that might add complexity compared to the adding reduce methods to the two classes. Thoughts ?

[robertwb]

I think this is good for now, and in general it'd be better to have more robust session (or at least main module) pickling. LGTM

[sb2nov]

Thanks. Yeah, having a more robust pickling story would be great.