WL#6969: Additional concurrency tests for BGC

Affects: Server-Prototype Only   —   Status: Complete   —   Priority: Medium

RATIONALE
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* Concurrency tests verify that there is not deadlock in group commit, non-group
commit and semi sync areas.

SUMMARY
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1. Concurrency test verifies that there is not deadlock on group commit when
semi-sync is disabled
2. Concurrency test verifies that there is not deadlock on group commit when
semi-sync is enabled
3. Concurrency test verifies that there is not deadlock on non-group commit when
semi-sync is enabled