[WIP] TopologyAwareHints: Take lock in HasPopulatedHints

[afcollins]

Prevent potential concurrent map access by taking a lock before reading the topology cache's hintsPopulatedByService map.

• staging/src/k8s.io/endpointslice/topologycache/topologycache.go (setHintsLocked, hasPopulatedHintsLocked): New helper functions. These are the same as the existing SetHints and HasPopulatedHints methods except that these helpers assume that a lock is already held.
  (SetHints): Use setHintsLocked.
  (HasPopulatedHints): Take a lock and use hasPopulatedHintsLocked. (AddHints): Take a lock and use setHintsLocked and hasPopulatedHintsLocked.
• staging/src/k8s.io/endpointslice/topologycache/topologycache_test.go (TestTopologyCacheRace): Add a goroutine that calls HasPopulatedHints.

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Comment /validate-backports to re-evaluate validity of the upstream PRs, for example when they are merged upstream.

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