Database level cache uses more memory than configured

Please include configurations and logs if available.

For confirmed bugs, please report:
- Version: 0.5.3, and all old versions
- Storage Backend: Does not matter
- Mixed Index Backend: Does not matter
- Link to discussed bug: https://groups.google.com/forum/?utm_medium=email&utm_source=footer#!msg/janusgraph-users/1PM-wkJpH-A/2pxrrlmSCQAJ & https://lists.lfaidata.foundation/g/janusgraph-users/topic/79704830#5525 (continued)

The google group link above provides details to reproduce.

Start with the reproduction details in the linked JanusGraph user discussions and compare the configured database-level cache size with the memory usage described there. Done means the confirmed reproduction no longer shows cache memory exceeding its configured limit, with regression coverage added where the relevant cache behavior is located.

Written by the indexing model from the issue text.