In previous design, there is not context.Context going through the frontend and the computation engine.
In 0.6, the computation engine introduces the context.Context.
So, I add the context.Context in the frontend and try to connect context.Context with modules -- metric and trace.
I am not sure the usage of the conext.Context in these modules is right. So I propose the PR.

The hierarchy of the context.Context:
```
rootCtx(context.Background)
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WithCancel()
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\|/
cancelMoServerCtx
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WithValue(configuration)
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moServerCtx
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----------------------------> context in metric, trace
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\|/
cancelRoutineCtx (every network connection has an independent context)
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------> WithCancel----->AuthenticateCtx (login authentication -- user, password, role)
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cancelRequestCtx (every request from the client has a new context)
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-------> WithCancel ------> LoadBatchCtx ( every batch has a independent context)
```

Approved by: @fengttt, @yingfeng, @aptend