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Default Win32 transport to WinHTTP and harden request lifetimes #752

Job	Run time
iOS (arm64-ios cross-compile)	1m 55s
Linux (x64-linux)	4m 53s
macOS (native)	2m 2s
Windows (x64-windows-static, WinHTTP)	3m 44s
Android (arm64-v8a API 23 cross-compile)	2m 31s
Windows (x64-windows-static, WinInet)	3m 58s
	19m 3s