Title: tcp_proxy_integration_test flakes at 1% rate.

Description:

The tcp_proxy_integration_test flakes at a low rate. With --runs_per_test=1000, I see 10 identical failures. All are a test failure where a different event from the expected half close event is received, followed by an assertion failure.

test/integration/tcp_proxy_integration_test.cc:137: Failure
Value of: fake_upstream_connection->waitForHalfClose()
Actual: false (Expected half close event, but got a different event.)
Expected: true
[2018-08-02 14:05:55.772][15][critical][assert] source/common/network/connection_impl.cc:71] assert failure: fd() == -1.

Repro steps:
bazel test //test/integration:tcp_proxy_integration_test --runs_per_test=1000