Update containerd to v1.2.9

[tao12345666333]

- What I did

Update containerd to v1.2.9. https://github.com/containerd/containerd/releases/tag/v1.2.9

The ninth patch release for containerd 1.2 provides a handful of bug fixes and an
update to the gRPC vendored codebase to include 3 CVE fixes provided in the upstream
v1.23.0 release of gRPC. Note that updating gRPC to the current release required small
changes to our core containerd codebase to match the upstream changes since gRPC v1.12.0.
These changes have been backported from containerd's master branch, as well as a
similar small change in ttrpc, requiring that package's vendoring to be updated.

In addition to the gRPC update to include CVE fixes, fixes were made to correct a
container's default Unix environment (introduced in 1.2.8), a small list of CRI plugin
fixes, as well as fixes for registry interactions where Docker-Content-Digest is not
returned (e.g. GitHub Package Registry), and a tar archive modification time bug found
by the buildkit maintainers. A fix to the zfs snapshotter was also included via a
re-vendoring of containerd's zfs import. More notes on these fixes are found below.

[tao12345666333]

Flaky test (tracked through #38521)

https://ci.docker.com/public/blue/rest/organizations/jenkins/pipelines/moby/branches/PR-39874/runs/1/nodes/49/steps/196/log/?start=0

[2019-09-06T03:37:10.170Z] 
[2019-09-06T03:37:10.170Z] ----------------------------------------------------------------------
[2019-09-06T03:37:10.170Z] FAIL: docker_cli_start_test.go:190: DockerSuite.TestStartReturnCorrectExitCode
[2019-09-06T03:37:10.170Z] 
[2019-09-06T03:37:10.170Z] docker_cli_start_test.go:200:
[2019-09-06T03:37:10.170Z]     c.Assert(exitCode, checker.Equals, 12, check.Commentf("out: %s", out))
[2019-09-06T03:37:10.170Z] ... obtained int = 125
[2019-09-06T03:37:10.170Z] ... expected int = 12
[2019-09-06T03:37:10.170Z] ... out: time="2019-09-06T03:37:09Z" level=error msg="error waiting for container: EOF" 
[2019-09-06T03:37:10.170Z] 
[2019-09-06T03:37:10.170Z] 

[thaJeztah]

LGTM, thanks!