WIP: Asynchronous send #393

Job	Run time
golangci-lint	1m 56s
test (stable)	1m 50s
test (1.24.x)	1m 44s
build-upload (amd64, linux, linux)	1m 27s
build-upload (amd64, mac, darwin)	1m 23s
build-upload (amd64, windows, windows)	1m 22s
	9m 42s

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