fix httptrace datarace

[t-yuki]

There is a data-race condition in xray/httptrace.go, because go's http client reuses idle connections aggressively.
DefaultTransport.getConn https://golang.org/src/net/http/transport.go#L990 dials new connection and reuses idleConn in parallel.

Without this change, the test will cause the below:

WARNING: DATA RACE
Write at 0x00c42051a7f8 by goroutine 129:
  github.com/aws/aws-xray-sdk-go/xray.(*Segment).Close()
      /home/t-yuki/go/src/github.com/aws/aws-xray-sdk-go/xray/segment.go:237 +0x248
  github.com/aws/aws-xray-sdk-go/xray.Capture.func1()
      /home/t-yuki/go/src/github.com/aws/aws-xray-sdk-go/xray/capture.go:23 +0x87
  github.com/aws/aws-xray-sdk-go/xray.Capture()
      /home/t-yuki/go/src/github.com/aws/aws-xray-sdk-go/xray/capture.go:48 +0x15a
  github.com/aws/aws-xray-sdk-go/xray.(*roundtripper).RoundTrip()
      /home/t-yuki/go/src/github.com/aws/aws-xray-sdk-go/xray/client.go:64 +0x1eb
  github.com/aws/aws-xray-sdk-go/xray.TestRoundTrip_reuse_datarace.func2()
      /home/t-yuki/go/src/github.com/aws/aws-xray-sdk-go/xray/client_test.go:215 +0x244

Previous read at 0x00c42051a7f8 by goroutine 32:
  github.com/aws/aws-xray-sdk-go/xray.(*HTTPSubsegments).ConnectStart()
      /home/t-yuki/go/src/github.com/aws/aws-xray-sdk-go/xray/httptrace.go:72 +0x6d
  github.com/aws/aws-xray-sdk-go/xray.NewClientTrace.func4()
      /home/t-yuki/go/src/github.com/aws/aws-xray-sdk-go/xray/httptrace.go:194 +0x69
  net.dialSingle()
      /home/t-yuki/go/tools/go/src/net/dial.go:537 +0xb43
  net.dialSerial()
      /home/t-yuki/go/tools/go/src/net/dial.go:515 +0x251
  net.(*Dialer).DialContext()
      /home/t-yuki/go/tools/go/src/net/dial.go:397 +0x8f1
  net.(*Dialer).DialContext-fm()
      /home/t-yuki/go/tools/go/src/net/http/transport.go:46 +0x99
  net/http.(*Transport).dial()
      /home/t-yuki/go/tools/go/src/net/http/transport.go:898 +0x322
  net/http.(*Transport).dialConn()
      /home/t-yuki/go/tools/go/src/net/http/transport.go:1143 +0x4ee
  net/http.(*Transport).getConn.func4()
      /home/t-yuki/go/tools/go/src/net/http/transport.go:957 +0x9f

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[luluzhao]

I merge this PR. Thank you again for your contribution.