fix(stream-generator): Split create/keep-alive streams routines

[periklis]

What this PR does / why we need it:
This pull request separates created/keep-alive routines in the stream generator. This enabled adding new streams at a different rate than keeping them alive.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
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