gRPC v1.7.2 is racey between tearDown and create sub-connection

[gyuho]

WARNING: 2017/11/10 23:44:48 grpc: addrConn.transportMonitor exits due to: grpc: the connection is closing
==================
WARNING: DATA RACE
Read at 0x00c4204899a8 by goroutine 196:
  google.golang.org/grpc.(*addrConn).tearDown()
      /go/src/github.com/coreos/etcd/gopath/src/google.golang.org/grpc/clientconn.go:1127 +0x232
  google.golang.org/grpc.(*ClientConn).Close()
      /go/src/github.com/coreos/etcd/gopath/src/google.golang.org/grpc/clientconn.go:812 +0x27c
  github.com/coreos/etcd/clientv3.TestBalancerDoNotBlockOnClose.func4()
      /go/src/github.com/coreos/etcd/gopath/src/github.com/coreos/etcd/clientv3/health_balancer_test.go:229 +0x5e

Previous write at 0x00c4204899a8 by goroutine 59:
  google.golang.org/grpc.(*ccBalancerWrapper).NewSubConn()
      /go/src/github.com/coreos/etcd/gopath/src/google.golang.org/grpc/balancer_conn_wrappers.go:174 +0x21b
  google.golang.org/grpc.(*balancerWrapper).lbWatcher()
      /go/src/github.com/coreos/etcd/gopath/src/google.golang.org/grpc/balancer_v1_wrapper.go:202 +0x197e

Goroutine 196 (running) created at:
  github.com/coreos/etcd/clientv3.TestBalancerDoNotBlockOnClose()
      /go/src/github.com/coreos/etcd/gopath/src/github.com/coreos/etcd/clientv3/health_balancer_test.go:227 +0x1d5
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:746 +0x16c

Goroutine 59 (running) created at:
  google.golang.org/grpc.(*balancerWrapperBuilder).Build()
      /go/src/github.com/coreos/etcd/gopath/src/google.golang.org/grpc/balancer_v1_wrapper.go:53 +0x42f
  google.golang.org/grpc.newCCBalancerWrapper()
      /go/src/github.com/coreos/etcd/gopath/src/google.golang.org/grpc/balancer_conn_wrappers.go:109 +0x2da
  google.golang.org/grpc.DialContext()
      /go/src/github.com/coreos/etcd/gopath/src/google.golang.org/grpc/clientconn.go:449 +0xa18
  google.golang.org/grpc.Dial()
      /go/src/github.com/coreos/etcd/gopath/src/google.golang.org/grpc/clientconn.go:340 +0x98
  github.com/coreos/etcd/clientv3.TestBalancerDoNotBlockOnClose()
      /go/src/github.com/coreos/etcd/gopath/src/github.com/coreos/etcd/clientv3/health_balancer_test.go:204 +0x4db
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:746 +0x16c
==================
WARNING: 2017/11/10 23:44:48 grpc: addrConn.transportMonitor exits due to: grpc: the connection is closing
WARNING: 2017/11/10 23:44:48 grpc: addrConn.transportMonitor exits due to: grpc: the connection is closing
INFO: 2017/11/10 23:44:48 ccBalancerWrapper: updating state and picker called by balancer: READY, 0xc42016a660
INFO: 2017/11/10 23:44:48 ccBalancerWrapper: removing subconn
--- FAIL: TestBalancerDoNotBlockOnClose (0.08s)
	testing.go:699: race detected during execution of test

Will close when upgrading gRPC to v1.7.3

c.f. grpc/grpc-go#1662, grpc/grpc-go#1665