implement message queue for WebChannels #965

Job	Run time
build	1m 40s
Julia 1.6 - ubuntu-latest - x64	4m 3s
Julia 1.6 - macOS-latest - x64	6m 40s
Julia 1.6 - windows-latest - x64	13m 24s
Julia 1 - ubuntu-latest - x64	3m 27s
Julia 1 - macOS-latest - x64	6m 45s
Julia 1 - windows-latest - x64	4m 54s
Julia nightly - ubuntu-latest - x64	6m 40s
Julia nightly - macOS-latest - x64	5m 30s
Julia nightly - windows-latest - x64	7m 5s
	1h 0m 8s

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