Backend-ai-agent

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- Fix repeating docker event polling even when there is connection/client-side
aiohttp errors.

- Upgrade aiohttp to v3.0 release.

- Improve dockerization. (55)

- Improve inner beauty.

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**NOTICE**

- From this release, the manager and agent versions will go together, which indicates
the compatibility of them, even when either one has relatively little improvements.

**CHANGES**

- Include the exit code of the last executed in-kernel process when returning
``build-finished`` or ``finished`` results in the batch mode.

- Improve logging to support rotating file-based logs.

- Upgrade aiotools to v0.5.2 release.

- Remove the image name prefix when reporting available images. (51)

- Improve debug-kernel mode to mount host-side kernel runner source into the kernel
containers so that they use the latest, editable source clone of the kernel runner.

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- Automatically assign the run ID if set None when starting a run.

- Pass environment variables in the start-config to the kernels via
``/home/work/.config/environ.txt`` file mounted inside kernels.

- Include the list of kernel images available to the agent when sending
heartbeats. (51)

- Remove simplejson from dependencies in favor of the standard library.
The stdlib has been updated to support all required features and use
an internal C-based module for performance.

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- Update aioredis to v1.0.0 release.

- Remove "mode" argument from completion RPC calls.

- Fix a bug when terminating overlapped execute streams, which has caused
indefinite hangs in the client side due to missing "finished" notification.

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- Implement virtual folder mounting (assuming /mnt is already configured)