Allow creation of multiple Cosmos containers

[mdrichardson]

Fixes #1975

Description

doOnce was preventing cosmosDbPartitionedStorage users from using multiple containers and/or databases. This is because doOnce acted on cosmosDbPartitionedStorage as a whole, rather than individual db/container combinations.

Specific Changes

• Store a dict of doOnce-awaited functions, keyed by db+container

Testing

Added two tests, which fail without this change and pass with it.

Considerations

Is this the right approach, or would it be better to store the container as a property in cosmosDbPartitionedStorage? My hunch is that there might be concurrency issues with the class property approach.

[coveralls]

Pull Request Test Coverage Report for Build 117639

• 6 of 6 (100.0%) changed or added relevant lines in 2 files are covered.
• 9 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.1%) to 79.293%

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Files with Coverage Reduction	New Missed Lines	%
libraries/botframework-streaming/src/webSocket/webSocketTransport.ts	2	80.65%
libraries/botframework-streaming/src/webSocket/webSocketServer.ts	7	66.67%

Totals
Change from base Build 117482:	-0.1%
Covered Lines:	9083
Relevant Lines:	11030

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💛 - Coveralls