Backend.ai-manager

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FIXES
- Legacy ZMQ interface: Revived a missing language parameter in legacy
client-side session token generation.
This has broken CodeOnWeb's PRACTICE page.
- Gateway: Increased timeouts when interacting with agents.
In particular, code execution timeouts must be longer than kernel execution
timeouts.
- Gateway: Added a missing transaction context during authorization.
This has caused "another operation in progress" errors with concurrent API
requests within a very short period of time (under a few tens of msec).

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CHANGES
- When launching a new kernel and accessing to an existing kernel, it scans
only "currently alive" instances by checking shadow keys that automatically
expires. This makes the Sorna service sustainable with abrupt agent failures.

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CHANGES
- When launching a new kernel, it now chooses the least loaded agent instead of
the first-found agent with free kernel slots.

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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.