Rework deletion e2e to use pluggable client interface to verify content is actually gone

[ncdc]

See #989 (comment)

"Pluggable client interface" so we can support both etcd and cockroachdb

[stevekuznetsov]

This has ... obvious drawbacks. :^)

[ncdc]

Updated to include crdb

[kcp-ci-bot]

Issues go stale after 90d of inactivity.
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/lifecycle stale

[kcp-ci-bot]

Stale issues rot after 30d of inactivity.
Mark the issue as fresh with /remove-lifecycle rotten.
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/lifecycle rotten

[kcp-ci-bot]

Rotten issues close after 30d of inactivity.
Reopen the issue with /reopen.
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/close