Deduplicate writer code

[MarkWolters]

There is a significant amount of code duplication among the Graph Index Writer implementations. Most of the common logic that could be abstracted to a common ancestor occurs in one of

• OnDiskSequentialGraphIndexWriter method write(Map<FeatureId, IntFunction<Feature.State>> featureStateSuppliers)
• OnDiskGraphIndexWriter method writeL0RecordsSequential(ImmutableGraphIndex.View view, Map<FeatureId, IntFunction<Feature.State>> featureStateSuppliers)
• NodeRecordTask method call()

The duplicate logic in question is the writing to disk of the L0 records in the graph including neighbors, features and padding.