Backend-ai-manager

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- Improve synchronization when executing codes right after creating kernels by
ensuring all DB operations (incl. read-only ops) to be inside (nested)
transactions.

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- Improve vfolder APIs to handle sub-directories correctly when uploading and use
the configured mount directory ("volumes/_mount" key in our etcd namespace).

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- Technical release to fix backend.ai-common depedency version.

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- Allow separate upgrade of the manager from v1.2 to v1.3 by extrapolating a new
"kernel_host" field in the return value of the internal krenel creation RPC call.

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- Now the Backend.AI gateway uses a modular architecture where you can add 3rd-party
extensions as aiohttp.web.Application and middlewares via ``BACKEND_EXTENSIONS``
environment variable. (65)

- Adopt aiojobs as the main coroutine task scheduler. (65)
Using this, improve handler/task cancellation as well.

- Public non-authorized APIs become accessible without "Date" HTTP header set. (65)

- Upgrade aiohttp to v3.0 release. (64)

- Improve dockerization support. (62)

- Fix "X-Method-Override" support that was interfering with RFC-7807-style error
reporting. Also return correct HTTP status code when failed route resolution.