Backend-ai-manager

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Hot-fix to add missing dependencies in requirements.txt and setup.py

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To avoid confusion with different version numbers in other Sorna sub-projects,
we skip the version 0.6.0 in all sub-projects.

**NEW**

- Implemented most of the REST API except streaming terminals and events.

- Added database schema for user/keypair information management.
It can be initialized using ``python -m sorna.gateway.models`` command.

**FIXES**

- Fixed duplicate kernel count decrementing when destroying kernels in legacy manager.

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**FIXES**

- Added a missing check for stale kernel sessions due to restarts of Sorna agents.
This bug has impacted public tutorial/workshops and demonstrations because the
manager does not recreate kernels at the right timing.

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**NEW**

- First public release.