[FEATURE] Try export to Neo4j Community Edition using UNIQUE constraints

[georgeh0]

Currently we only support to export to Neo4j Enterprise Edition, because Community Edition doesn't support KEY constraints.

Previously, we tried to use UNIQUE constraints for Community Edition, but it turns out to be unstable.

We should have a try on the latest version of Neo4j Community Edition with UNIQUE constraint and see how things go, and enable the support if feasible.

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[georgeh0]

Tested on community version (#983). After changing KEY constraint to UNIQUE constraint, it just works stably.