Support CorrelationContext in python agent

[alonelaval]

Please answer these questions before submitting your issue.

• Why do you submit this issue?
• Question or discussion
• Bug
• Requirement
• Feature or performance improvement

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Question

• What do you want to know?

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Bug

• Which version of SkyWalking, OS and JRE?

• Which company or project?

• What happen?
  If possible, provide a way for reproducing the error. e.g. demo application, component version.

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Requirement or improvement

• Please describe about your requirements or improvement suggestions.