Register derived parallel callbacks

[knelli2]

Proposed changes

The GlobalCache never really gets pupped, but it is sized by the MemoryMonitor. Therefore, all it's members need to be registered with charm. Before the derived callbacks weren't registered, so we got the Unrecognized PUP::able::PUP_ID error. This PR will register the callbacks only once so that doesn't happen.

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Code review checklist

• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
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• The code follows the stylistic and code quality guidelines listed in the
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• The PR lists upgrade instructions and is labeled bugfix or
  new feature if appropriate.

Further comments

[wthrowe]

Wrong class name in the commit message. (Should be MockDistributedObjectProxy.)