Fixing TSAN data races of the kind -
https://source.cloud.google.com/results/invocations/c3f02253-0d0b-44e6-917d-07e4bc0d3d62/targets/%2F%2Ftest%2Fcpp%2Fperformance:writes_per_rpc_test@poller%3Dpoll/log
I think the original issue was the non-atomic load in
writes_per_rpc_test.cc but also making the change to use barriers
instead of relaxed atomics (probably unimportant but I'll prefer safety
in the absence of otherwise comments).
Could probably also use std::atomic but feeling a bit lazy to make the
changes throughout.
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625b3544e7