breaking(go): update flow streaming protocol to SSE

[hugoaguirre]

OK

curl -X POST -H "Accept: text/event-stream" http://localhost:3400/streamy -d '{"data":5}'
data: {"count":0}\n\n
data: {"count":1}\n\n
data: {"count":2}\n\n
data: {"count":3}\n\n
data: {"count":4}\n\n
data: {"result": "done: 5, streamed: 5 times"}\n\n

ERROR

curl -X POST -H "Accept: text/event-stream" http://localhost:3400/streamy -d '{"data":5}'                                                                       
data: {"message": {"count":0}}\n\n
data: {"message": {"count":1}}\n\n
data: {"message": {"count":2}}\n\n
data: {"message": {"count":3}}\n\n
data: {"message": {"count":4}}\n\n
data: {"error": {"status": "INTERNAL", "message": "stream flow error", "details": "this is a dummy error"}}\n\n         

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[hugoaguirre]

Go test cases failing due to unrelated errors, fix can be found here: #1595