Disable vertex cache for cardinality checks

[cdegroc]

In a service running JanusGraph embedded, I observed a number of sizeable allocations coming from JanusGraphVertexFeatures#getCardinality.

From a profile (included below), it looks like a StandardJanusGraphTx transaction is created when checking a vertex property's cardinality. The transaction itself allocates a CaffeineVertexCache. The CaffeineVertexCache internally creates a NonBlockingHashMap which initial size is configured in the GraphDatabaseConfiguration class or the graph's dynamic configuration.

I think we don't need a vertex cache for cardinality checks and would suggest disabling it to release some memory.

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[cdegroc]

Great finding, thank you!

btw why is this PR marked as WIP?

I wanted to confirm the problem before creating an issue and a proper PR. I'm fine with merging as-is if that's good enough.

[cdegroc]

I usually have a default vertex cache size and dirty vertex size set to 0 and set a proper value per transaction when needed, thus I haven't bumped into this issue before. However, the solution here perfectly makes sense to me as the transaction is closed immediately after we exiting this method. We don't create any new mutations, thus dirty vertex size should be 0 and read-only transaction make sense. We return a single property key, and don't reuse this transaction for anything else, thus vertex cache size 0 makes sense here. LGTM. Thank you @cdegroc !

Thank you so much for confirming @porunov 🙇