Race condition on cluster creation/deletion causes unexpected behaviour

[jgwest]

See details on parent issue.

The vast majority of this PR is test changes, the new files in pkg/utils are the only substantial changes to product code.

This PR:

• Forks a few of the util/db/cluster.go functions from Argo CD, and replaces their use of the race-condition-prone ApplicationLister, with a simple kubeclient query.
• Forks the test cases that test those functions, as well, to make sure nothing has regressed.
• As I state in the PR, ideally we would upstream these changes to Argo CD in some form, but in the mean time (and until Argo CD has adopted the fix in all the versions we support), this fixes the issue here.

Fixes #176

[OmerKahani]

What would be the steps once ArgoCD adopts the fix? Do we revert this PR?

[jgwest]

@OmerKahani Good question, what we need from Argo CD is a version of this API (in the Argo CD code) that doesn't use the Secret lister (which can have delayed results that cause the race condition), but instead just query the cluster directly.

Once this is in place in Argo CD, we would pull in that new version of Argo CD as a dependency, and switch to using that API. At that point, we would:

• Delete pkg/utils/cluster*.go (no longer needed)
• Replace references to "github.com/argoproj-labs/applicationset/pkg/utils" with the new Argo CD package