Fix the handling of removing disconnected streams to avoid a panic when multiple streams disconnect from the sdkserver

[roberthbailey]

What type of PR is this?

Uncomment only one /kind <> line, press 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
/kind release

What this PR does / Why we need it: Fixes a runtime panic when multiple streams disconnect from the sdkserver. This happens when a game server creates more than one watch and then closes them all at the same time. Normally, watches last for the lifetime of a game server, but if a game server intentionally disconnects watches and/or expects to be able to exit and restart (which implicitly disconnects the watches) then we hit this edge case.

Which issue(s) this PR fixes:

Closes #3640

Special notes for your reviewer: I tested this fix two ways:

1. The new unit test fails with the old code paths with the error:

--- FAIL: TestSDKServer_SendGameServerUpdateRemovesDisconnectedStream (2.01s)
panic: runtime error: slice bounds out of range [2:1] [recovered]
	panic: runtime error: slice bounds out of range [2:1]

2. I installed the fix into a test cluster and verified that the nodejs sample program that triggered the panic previously no longer does so.

[agones-bot]

Build Succeeded 👏

Build Id: 63cdec2d-ffa5-42c9-b907-f0f06174ffa2

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

• image: us-docker.pkg.dev/agones-images/ci/agones-controller:1.39.0-dev-4dd7800-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-extensions:1.39.0-dev-4dd7800-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-sdk:1.39.0-dev-4dd7800-linux-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-ping:1.39.0-dev-4dd7800-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-allocator:1.39.0-dev-4dd7800-amd64
• Linux C++ SDK (build): agonessdk-1.39.0-dev-4dd7800-amd64-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.39.0-dev-4dd7800-amd64.zip

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

• https://4dd7800-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/3668/head:pr_3668 && git checkout pr_3668
• helm install agones ./install/helm/agones --namespace agones-system --set agones.image.registry=us-docker.pkg.dev/agones-images/ci --set agones.image.tag=1.39.0-dev-4dd7800-amd64

[zmerlynn]

I'm ok either way with the below comment - leave it if you wish. :)