Keep spans active on AsyncHandler#onStream

[milanvdm]

What does this PR do?

Currently, when using a custom StreamedAsyncHandler with the AsyncHttpClient, there is no active span in the onStream method.
This PR makes sure that the span is activated when entering onStream

Related to #1172

Checklist

• My code follows the style guidelines of this project
• I have rebased my changes on top of the latest master branch

• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

• I have updated CHANGELOG.asciidoc
• I have updated supported-technologies.asciidoc
• Added an API method or config option? Document in which version this will be introduced
• Added an instrumentation plugin? How did you make sure that old, non-supported versions are not instrumented by accident?

[ghost]

💚 Build Succeeded

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Build stats

• Build Cause: [Branch indexing]

• Start Time: 2020-05-10T21:50:22.789+0000

• Duration: 37 min 17 sec (2237388)

• Commit: ec2b7eb

Test stats 🧪

Test	Results
Failed	0
Passed	1261
Skipped	12
Total	1273