Review datastore backing types

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Some issues emerged due to backward compatibility with earlier Neo4j version.

Issue: The dotnet "decimal" type is currently stored as a fixed-point "long" in Neo4j.
Solution: Add a feature to switch the behavior so decimals can optionally get stored as floating point numbers

Issue: The dotnet "DateTime" type is currently stored as a epoch "long" in Neo4j.
Solution: Add a feature to switch the behavior so DateTime can optionally get stored as "Neo4j DateTime" numbers

Issue: The dotnet "TimeSpan" type is currently unsupported.
Issue: The dotnet "Point" type is currently unsupported.

Issue: How could we change the default (for DateTime for example) without causing trouble for existing databases?