fix: make useNamedPipe idempotent if already connected to pipeName

[stevengum]

Fixes #3449

Specific Changes

• Make BotFrameworkAdapter.useNamedPipe idempotent by checking for StreamingServer and new namedPipeName member
• Add private namedPipeName?: string member to BotFrameworkAdapter
• Minor docstring fix in botframework-streaming.NamedPipeServer

Testing

Add new tests for new behavior in useNamedPipe

[coveralls]

Pull Request Test Coverage Report for Build 687760863

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 47 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.03%) to 84.984%

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Files with Coverage Reduction	New Missed Lines	%
libraries/botbuilder-lg/src/templateExtensions.ts	1	84.48%
libraries/adaptive-expressions/src/triggerTrees/trigger.ts	2	61.92%
libraries/adaptive-expressions/src/triggerTrees/node.ts	8	91.7%
libraries/botbuilder/src/botFrameworkAdapter.ts	36	89.09%

Totals
Change from base Build 687729579:	-0.03%
Covered Lines:	18816
Relevant Lines:	21083

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💛 - Coveralls