[#6591] Concurrency issue with TimeZoneConverter

[ceciliaavila]

Fixes #6591

Description

This PR fixes the concurrency issue in the TimeZoneConverter class by changing the Dictionaries to be thread-safe and also adding a static constructor to the class to execute the LoadData method only once.

Specific Changes

• Updated TimeZoneConverter class adding a static constructor and using ConcurrentDictionary.

Testing

Here we can see concurrent tests failing before the changes and the same tests passing after the changes.

[ceciliaavila]

Hi @tracyboehrer, this is the fix for the concurrency issue.

[coveralls]

Pull Request Test Coverage Report for Build 341679

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 5 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.003%) to 79.106%

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Files with Coverage Reduction	New Missed Lines	%
/libraries/AdaptiveExpressions/BuiltinFunctions/GetNextViableTime.cs	1	90.91%
/libraries/integration/Microsoft.Bot.Builder.Integration.AspNet.Core/ServiceCollectionExtensions.cs	1	93.55%
/libraries/Microsoft.Bot.Connector.Streaming/Application/TimerAwaitable.cs	1	68.25%
/libraries/AdaptiveExpressions/TimeZoneConverter.cs	2	88.0%

Totals
Change from base Build 340498:	-0.003%
Covered Lines:	25829
Relevant Lines:	32651

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💛 - Coveralls

[johnataylor]