Add lock to to sdk-conformance compare()

[markmandel]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespace from that line:

/kind breaking
/kind bug

/kind cleanup

/kind documentation
/kind feature
/kind hotfix

What this PR does / Why we need it:

Likely isn't an issue, but figure it wouldn't hurt, and probably should be there anyway.

Which issue(s) this PR fixes:

N/A

Special notes for your reviewer:
N/A

[agones-bot]

Build Failed 😱

Build Id: 49836412-313c-4b78-8c70-4cf5eb2fc75b

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[agones-bot]

Build Succeeded 👏

Build Id: 84b601ae-7f27-4484-a46e-686fcb024a89

The following development artifacts have been built, and will exist for the next 30 days:

• image: gcr.io/agones-images/agones-controller:1.9.0-1e3c771
• image: gcr.io/agones-images/agones-sdk:1.9.0-1e3c771
• image: gcr.io/agones-images/agones-ping:1.9.0-1e3c771
• Linux C++ SDK (build): agonessdk-1.9.0-1e3c771-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.9.0-1e3c771.zip

A preview of the website (the last 30 builds are retained):

• https://1e3c771-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/1801/head:pr_1801 && git checkout pr_1801
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.9.0-1e3c771

[aLekSer]

Agree that this mutex could help.

[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aLekSer, markmandel

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