Fixed the memory consumption when creating a task

[azhavoro]

Fixed the memory consumption when creating a task

Motivation and context

Currently, the task creation process can consume too much memory, especially for large videos.

How has this been tested?

Manually + unittests

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG file
  - [ ] I have updated the documentation accordingly
  - [ ] I have added tests to cover my changes
  - [ ] I have linked related issues (read github docs)
  - [ ] I have increased versions of npm packages if it is necessary (cvat-canvas,
  cvat-core, cvat-data and cvat-ui)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2020 Intel Corporation
#
# SPDX-License-Identifier: MIT

[coveralls]

Coverage increased (+0.01%) to 62.215% when pulling 3e8e397 on az/fix_memory_consumption into 6dd8130 on develop.