Avoid growing notifications map during recovery

As this could negatively affect recovery performance of long logs.

Proposed Changes

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Types of Changes

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[ ] Bug fix (non-breaking change which fixes issue #NNNN)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
[ ] Documentation (correction or otherwise)
[ ] Cosmetics (whitespace, appearance)

Checklist

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[ ] I have read the CONTRIBUTING.md document
[ ] I have signed the CA (see https://cla.pivotal.io/sign/rabbitmq)
[ ] All tests pass locally with my changes
[ ] I have added tests that prove my fix is effective or that my feature works
[ ] I have added necessary documentation (if appropriate)
[ ] Any dependent changes have been merged and published in related repositories

Further Comments

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rabbitmq / ra