Use async/await in CKEditorContext#_destroyContext() to fix the destruction process

[Mgsy]

Suggested merge commit message (convention)

Fix: Use async/await in CKEditorContext#_destroyContext() to handle context destruction properly. Closes #283.

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It is not that easy to test, so here is a short guide explaining how to do it:

1. Get the real-time-collaboration-comments-outside-of-editor-for-react collaboration sample and replace original src/sample.js with my code.
2. In src/sample.js add import path to the local ckeditor5-react package.
3. Build and run the sample.
4. Open the dev tools and the network tab.
5. Click the button to create editors.
6. The WebSocket connection should be pending.
7. Click the button to destroy editors.
8. The WebSocket connection should be closed.

The better description and GIFs are available in the original issue.

[coveralls]

Pull Request Test Coverage Report for Build 376

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 366:	0.0%
Covered Lines:	152
Relevant Lines:	152

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💛 - Coveralls

[pomek]

Post-merge LGTM.