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kubenet: yield lock while executing CNI plugin.

The CNI plugin can take up to 3 seconds to execute. CNI plugins can safely be
executed in parallel, so yield the lock to speed up pod creation.

This caused problems with the pod latency tests - previously, CNI plugins executed
in under 20ms. Now they must wait for DAD to finish and addresses to leave
tentative state.

Fixes: #54651

**What this PR does / why we need it**:
After upgrading CNI plugins to v0.6 in #51250, the pod latency tests began failing. This is because the plugins, in order to support IPv6, need to wait for DAD to finish. Because this
delay is while the kubenet lock is held, it significantly slows down the pod creation rate.

**Special notes for your reviewer**:
The CNI plugins also do locking for their critical paths, so it is safe to run them concurrently.

**Release note**:
```release-note
NONE
```