Simplify CI jobs

[lalitpagaria]

Current CI job have following issues -

• Frequent OOM
• Only test Linux build
• Only text Python 3.7 build
• Missing coverage run
• Missing mypy on rest_api module
• PR feedback about code formatting is very slow

I have following suggestions -

1. Use solution similar to Azure pipeline to test build on Linux, MacOS and Windows (Also we need to tag some tests with env as well, like faiss tests may fail on Windows so with tag we can disable them)
2. Use Tox to test build on all python supported versions (3.6, 3.7, 3.8 and 3.9)
3. Use Black or Flake8 along with mypy and pylint, static code checker in separate CI job to get fast code feedback
4. Use coverage to collect code coverage information via pytest (for first we will not block merge, but later merge can me blocked if PR does not have good code coverage)
5. Move slow tests to separate CI jobs to mitigate OOM issue.
6. Enable parallel test run

Please correct or suggest better options as well.

[lalitpagaria]

@tholor There are steady increase of downloads from Windows users. I think I would be good idea setup CI on Windows platform as well.

[lalitpagaria]

Same also true for downloads for different python versions as well.

[lalitpagaria]

Adding article about how @jina-ai done it.

https://hanxiao.io/2021/01/24/Speedup-CI-Workflow-in-Github-Actions-via-Strategy-Matrix/

[lalitpagaria]

After adding MilvusDocumentStore number of tests and test run time increased significantly. We would need fast feedback CI pipeline -

[oryx1729]

resolved by #825