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parent fb1e2326ce076e0571053e7598e863f4045df470
author jaehwan0214 <60352009+jaehwan0214@users.noreply.github.com> 1602660817 +0900
committer GitHub <noreply@github.com> 1602660817 +0900
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[NEMO-392] Support combine in streaming (#299)

JIRA: [NEMO-392: Support combine in streaming] (https://issues.apache.org/jira/projects/NEMO/issues/NEMO-392)

**Major changes:**
- Added GBKTransform class to support stream processing for GroupByKey and Combine.Perkey operation.
     - Each time an element arrives, GBKTransform invokes runner to process a single element and stores its state,       instead of waiting to process it until the time to emit it to downstream.
- Removed GroupByKeyAndWindowDoFnTransform since GBKTransform can support both Combine.Perkey 
   and GroupByKeyAndWindow operation.

**Minor changes to note:**
- N/A

**Tests for the changes:**
- Added GBKStreamingTransformTest.

**Other comments:**
- Most of the work is done by TaeGun Um

Closes #299