fix sub-subsegment datarace

[t-yuki]

There is a data-race condition in xray.BeginSubsegment function, the line seg.ParentSegment.totalSubSegments++ because seg.ParentSegement isn't locked well for deeply nested segments.

Without this change, the test will cause the below:

=== RUN   TestSubsegmentDataRace
==================
WARNING: DATA RACE
Read at 0x00c42037f218 by goroutine 93:
  github.com/aws/aws-xray-sdk-go/xray.BeginSubsegment()
      /home/t-yuki/go/src/github.com/aws/aws-xray-sdk-go/xray/segment.go:188 +0x408
  github.com/aws/aws-xray-sdk-go/xray.TestSubsegmentDataRace.func1()
      /home/t-yuki/go/src/github.com/aws/aws-xray-sdk-go/xray/segment_test.go:40 +0xcf

Previous write at 0x00c42037f218 by goroutine 95:
  github.com/aws/aws-xray-sdk-go/xray.BeginSubsegment()
      /home/t-yuki/go/src/github.com/aws/aws-xray-sdk-go/xray/segment.go:188 +0x42a
  github.com/aws/aws-xray-sdk-go/xray.TestSubsegmentDataRace.func1()
      /home/t-yuki/go/src/github.com/aws/aws-xray-sdk-go/xray/segment_test.go:39 +0x98

Goroutine 93 (running) created at:
  github.com/aws/aws-xray-sdk-go/xray.TestSubsegmentDataRace()
      /home/t-yuki/go/src/github.com/aws/aws-xray-sdk-go/xray/segment_test.go:37 +0x1f9
  testing.tRunner()
      /home/t-yuki/go/tools/go/src/testing/testing.go:746 +0x16c

Goroutine 95 (running) created at:
  github.com/aws/aws-xray-sdk-go/xray.TestSubsegmentDataRace()
      /home/t-yuki/go/src/github.com/aws/aws-xray-sdk-go/xray/segment_test.go:37 +0x1f9
  testing.tRunner()
      /home/t-yuki/go/tools/go/src/testing/testing.go:746 +0x16c
==================

Also I included a test tweak for TestCaptureAsync to suppress undesirable data-race detection.

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

[luluzhao]

I merged this PR. Thank you for your contribution.