Update some str methods for recent SDK representation changes

Update Operation.str() to reflect that receivers is now an array
instead of a defaultdict.

Update Worker* objects to not output serialized combine_fn.

These problems surface only when logging at DEBUG level.
----Release Notes----
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=117727480

Small fixes in BigQuery snippets and wordcount example.