Improved the performance of the ImageLoadObserver

[mlewand]

Suggested merge commit message (convention)

Fix: Improved markup operation performance of the editor with the image plugin enabled. Closes ckeditor/ckeditor5#5766.

---

Additional information

Do we want to add a test for that? I feel that integration test for this performance improvement would be a nice protection.

---

Example verification benchmarks:

• base (master branch): 11.01s to remove attributes
• this branch: 2.33s to remove attributes

Note that this was performed on a idling, real environment notebook, so other factor might distort the result. But it's in line with what you feel manually checking it, so it gives the right idea.

Markup used in the editor can be found here: https://gist.github.com/mlewand/109ae7e518cb91d90318c81331b7d37a

The further performance could be addressed by tweaking engine package.

[coveralls]

Coverage decreased (-0.1%) to 99.698% when pulling 07b1b28 on i/4504 into 11c9f39 on master.

[Reinmar]

Do we want to add a test for that? I feel that integration test for this performance improvement would be a nice protection.

It'd be nice but we've got too many problems with those tests. They are quite unstable, especially on CI. So, I'd skip that.