[GraphBolt] Keep references valid in CachePolicy.

[mfbalin]

Description

After we switched the hash table we use and the optimizations we applied such as passing pointers to CacheKey objects around, we needed to make sure that these pointers and references stay valid by utilizing data structures that guarantee to keep them valid.

Checklist

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [$CATEGORY] (such as [NN], [Model], [Doc], [Feature]])
• I've leverage the tools to beautify the python and c++ code.
• The PR is complete and small, read the Google eng practice (CL equals to PR) to understand more about small PR. In DGL, we consider PRs with less than 200 lines of core code change are small (example, test and documentation could be exempted).
• All changes have test coverage
• Code is well-documented
• To the best of my knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change
• Related issue is referred in this PR
• If the PR is for a new model/paper, I've updated the example index here.

Changes

[dgl-bot]

To trigger regression tests:

• @dgl-bot run [instance-type] [which tests] [compare-with-branch];
  For example: @dgl-bot run g4dn.4xlarge all dmlc/master or @dgl-bot run c5.9xlarge kernel,api dmlc/master

[dgl-bot]

Commit ID: 07495c8

Build ID: 1

Status: ⚪️ CI test cancelled due to overrun.

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Full logs path: link

[dgl-bot]

Commit ID: 7a4f446

Build ID: 2

Status: ✅ CI test succeeded.

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[dgl-bot]

Commit ID: 09653e6

Build ID: 3

Status: ✅ CI test succeeded.

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Full logs path: link

[frozenbugs]

LGTM