Migrate monitoring VariableHistoryIntervalStore base class to ConcurrentDictionary

[cquirosj]

Addresses:

• https://github.com/Particular/ServiceControl/issues/4458

Although the issue described in #4458 is not reproducible, this PR is an improvement to the code that could have made the problem happen.

Also, while trying to reproduce this bug, it was discovered that the lines of code that triggered the exception did not have test coverage.

PoA

• [x] Migrate dictionary variables in VariableHistoryIntervalStore to ConcurrentDictionary
• [x] Add test coverage to message metrics aggregation