Test that different extended data streams within a channel are
thread-safe.
benchmark old MB/s new MB/s speedup
BenchmarkEndToEnd 79.26 87.98 1.11x
benchmark old allocs new allocs delta
BenchmarkEndToEnd 110 73 -33.64%
benchmark old bytes new bytes delta
BenchmarkEndToEnd 2605720 1299768 -50.12%
LGTM=dave, jpsugar
R=agl, dave, jpsugar
CC=golang-codereviews
https://golang.org/cl/136420043