Backend.ai-manager

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- Support CORS (cross-origin resource sharing) for browser-based API clients (99).
- Fix the agent revival detection routine to update agent's address and region
for movable demo devices (100).
- Update use of deprecate APIs in our dependencies such as aiohttp and aiodocker.
- Let the config server to refresh configuration values from etcd once a minute.

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- Expanded virtual folder APIs
- Downloading and uploading large files from virtual folders via streaming (70)
- Inviting other users and accepting such invitations with three-level permissions
(read-only, read-write, read-write-delete) for collaboration via virtual folders
(80)
- Now it requires explicit "recursive" option to remove directories (89)
- New "mkdir" API to create empty directories (89)
- Support listing files in the session's main container. (63)
- All API endpoints are now available *without* version prefixes, as we migrate
to the vanilla aiohttp v3.4 release. (78)
- Change `user_id` column type of `keypairs` model from integer to string.
Now it can be used to store the user emails, UUIDs, or whatever identifiers
depending on the operator's environment.

Clients must be upgrade to 1.3.7 or higher to use string `user_id` properly.
(The client will auto-detect the type by trying type casting.)

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- Add CORS support (Hotfix 99 backported from v1.4 and master)

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- Drop custom-patched aiohttp and update it to official v3.3 release. (78)
- Fix intermittent failures in streaming uploads of small files.
- Fix an internal "infinity integer" representation to have correct 64-bit maximum
unsgined value.

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- Fix a regression bug when restarting kernels.

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- Limit the default number of worker processes to avoid unnecessarily many workers in
many-core systems and database connection exhaustion errors (lablup/backend.ai17)
- Upgrade aiotools to v0.6.0 release.
- Ensure aiohttp's shutdown handlers to have access to databases during their
execution, by moving connection pool cleanups to the aiohttp's cleanup handler.