Rework org.eclipse.core.internal.utils.Cache with standard java

[jukzi]

• Threadsafe implementation
• Instead of predefined cache size use SoftReferences.

Should avoid random NullPointerException in concurrent use: https://github.com/eclipse-platform/eclipse.platform/issues/1411

[github-actions[bot]]

Test Results

 1 734 files   -  3   1 734 suites   - 3   1h 42m 17s :stopwatch: + 15m 16s  3 966 tests  -  6   3 944 :white_check_mark:  -  5   22 :zzz: ±0  0 :x:  - 1  12 495 runs   - 18  12 334 :white_check_mark:  - 17  161 :zzz: ±0  0 :x:  - 1 

Results for commit afa0d18a. ± Comparison against base commit 309a7f0d.

This pull request removes 6 tests.

``` AutomatedResourceTests AllUtilsTests CacheTest ‑ testBasic AutomatedResourceTests AllUtilsTests CacheTest ‑ testBasic2 AutomatedResourceTests AllUtilsTests CacheTest ‑ testCacheLimit AutomatedResourceTests AllUtilsTests CacheTest ‑ testDiscardAll AutomatedResourceTests AllUtilsTests CacheTest ‑ testDiscardHead AutomatedResourceTests AllUtilsTests CacheTest ‑ testUpdate ```

:recycle: This comment has been updated with latest results.

[jukzi]

To make sure the memory does not explode by this change i took heapdump on platform workspace during clean build. It looks really harmless (80kb):