Avoid deadlock in RealMutableStore

[amrfarid140]

Closes #657

Description

The change ensures that the store lock is always released from withThreadSafety regardless of the execution result of threadSafety.block().

This helps in avoiding the lock being in a locked state when a cancellation exception is propagated from downstream.

The test mentioned in the issue and added here now passes 100% of the time. Of course happy to delete it (or move it somewhere else) before merging.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Test Plan

See unit test

Checklist:

Before submitting your PR, please review and check all of the following:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my change is effective
• New and existing unit tests pass locally with my changes

Additional Notes:

Add any other information about the PR here.