Backend.ai-manager

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Fixes
* Remove unwanted empty key while setting etcd config with nested dict. ([275](https://github.com/lablup/backend.ai-manager/issues/275))
* Set correct scaling group name during session scheduling. ([276](https://github.com/lablup/backend.ai-manager/issues/276))
* Destroying sessions now returns periodically collected statistics instead of last-moment statistics ([279](https://github.com/lablup/backend.ai-manager/issues/279))
* Apply batching to statistics synchroniztion to minimize time used inside DB transactions by batching and splitting transaction blocks ([280](https://github.com/lablup/backend.ai-manager/issues/280))
* Reduce potential DB transaction deadlocks during scheduling of many (~hundreds) pending sessions ([281](https://github.com/lablup/backend.ai-manager/issues/281))

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Fixes
* Missing concurrency update for self-terminated sessions (regression of [269](https://github.com/lablup/backend.ai-manager/issues/269)) ([#274](https://github.com/lablup/backend.ai-manager/issues/274))

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Features
* Add shared memory column to resource preset ([267](https://github.com/lablup/backend.ai-manager/issues/267))

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Features
* Add API to rename a file/directory inside a virtual folder. ([266](https://github.com/lablup/backend.ai-manager/issues/266))
* Exclude TERMINATING sessions from user resource accounting to allow spawning new sessions when there are stuck-in-TERMINATING sessions ([269](https://github.com/lablup/backend.ai-manager/issues/269))

Fixes
* Prevent destruction of kernels in PREPARING/PULLING statuses ([270](https://github.com/lablup/backend.ai-manager/issues/270))

Miscellaneous
* Adopt towncrier for changelog management (for 19.09 series) ([271](https://github.com/lablup/backend.ai-manager/issues/271))

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* NEW: Add "forced" option to the session destruction API ([250](https://github.com/lablup/backend.ai-manager/issues/250))
* NEW: Backport "etcd forget-image" manager CLI command from the master and add "forget_image" GraphQL
mutation for super-admins.
* IMPROVE: Support project/repository/tag pagination in rescan-images for Docker registries with a large
number of images and tags ([252](https://github.com/lablup/backend.ai-manager/issues/252))
* FIX: Strip owner-access-key argument when checking API params for input validation
with query parameters ([251](https://github.com/lablup/backend.ai-manager/issues/251))
* FIX: Potential SQL syntax error during synchronizing real-time statistics from Redis to DB caused when
no statistics data is available.
* FIX: Add missing privilege checks for some of GraphQL mutation APIs and improve logging for GraphQL
queries and mutations using graphene middleware. ([254](https://github.com/lablup/backend.ai-manager/issues/254))

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* MAINTENANCE: Update backend.ai-common dependency.