Prepare chunks on the fly

[Marishka17]

Motivation and context

Currently, when creating a task, original and compressed chunks are prepared.
This takes a significant amount of time and consumes a lot of disk space.
This PR contains a new way to create a task.
At the time of task creation minimum necessary information about the loaded data is collected.
(mapping of key frames and timestamps for video, dummy chunks for images).
Chunks are created "on the fly" when a request for needed chunk is received.
Created chunks are stored in a temporary cache of limited size.

How has this been tested?

Manually

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG file
• I have added tests to cover my changes
• 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]

Pull Request Test Coverage Report for Build 7244

• 242 of 266 (90.98%) changed or added relevant lines in 8 files are covered.
• 185 unchanged lines in 6 files lost coverage.
• Overall coverage increased (+0.2%) to 69.931%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
cvat/apps/engine/models.py	11	12	91.67%
cvat/apps/engine/media_extractors.py	7	10	70.0%
cvat/apps/engine/task.py	55	58	94.83%
cvat/apps/engine/frame_provider.py	16	20	80.0%
cvat/apps/engine/prepare.py	102	115	88.7%

Files with Coverage Reduction	New Missed Lines	%
src/annotations.js	5	48.63%
cvat/apps/authentication/serializers.py	6	56.52%
src/api-implementation.js	8	76.32%
src/annotations-history.js	14	30.77%
src/api.js	28	52.44%
src/session.js	124	53.61%

Totals
Change from base Build 7126:	0.2%
Covered Lines:	11982
Relevant Lines:	16670

---

💛 - Coveralls

[nmanovic]

@Marishka17 , need to fix a couple of codacy issues.

[bsekachev]

Client part LGTM. I have only one question. Should we enable the option useCache by default if it doesn't work for all videos?

[Marishka17]

@bsekachev , ok i will change default value.

[nmanovic]

@Marishka17 , need to resolve conflicts.
@azhavoro , could you please comment or approve the PR?

[azhavoro]

LGTM, but need to fix a couple of minor comments and add some tests.

[chaoyifei]

HI,When does this pr merge

[nmanovic]

I have merged the PR with some minor modifications manually.