Switching to use SpanConcurrentMap in the concurrency plugin

[eyalkoren]

What does this PR do?

Implementing #2092 for the concurrency plugin

Checklist

• This is an enhancement of existing features, or a new feature in existing plugins
  • I have updated CHANGELOG.asciidoc
  • I have added tests that prove my fix is effective or that my feature works

[apmmachine]

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Build stats

• Start Time: 2021-10-11T07:42:56.671+0000

• Duration: 62 min 58 sec

• Commit: 674e00a

Test stats 🧪

Test	Results
Failed	0
Passed	2420
Skipped	19
Total	2439

💚 Flaky test report

Tests succeeded.

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