Swap to using CoGBK as grouping primitive instead of GBK

The intent is for the semantics of both GBK and CoGBK to be
unchanged, just swapping their status as primitives.

CoGBK is a more powerful operator then GBK allowing for two key benefits:

1) SDKs are simplified: transforming a CoGBK into a GBK is trivial while the reverse is not.
2) It will be easier for runners to provide more efficient implementations of CoGBK as they will be responsible for the logic which takes their own internal grouping implementation and maps it onto a CoGBK.

This requires the following modifications to the Beam code base:

1) Make GBK a composite transform in terms of CoGBK.
2) Move the CoGBK from contrib to runners-core as an adapter*. Runners that more naturally support GBK can just use this and everything executes exactly as before.

*just like GroupByKeyViaGroupByKeyOnly and UnboundedReadFromBoundedSource

Imported from Jira [BEAM-490](https://issues.apache.org/jira/browse/BEAM-490). Original Jira may contain additional context.
Reported by: lcwik.

Start by tracing the existing GBK and CoGBK implementations, including the contrib and runners-core locations named in the issue. Verify how GBK can become a composite of CoGBK, how the adapter preserves both transforms' semantics, and that runners can continue using the existing grouping behavior.

Written by the indexing model from the issue text.