Lucene.Net.Search.ReferenceContext<T>: Converted to ref struct and reworked TestControlledRealTimeReopenThread.TestStraightForwardDemonstration() to verify functionality

[NightOwl888]

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Summary of the changes (Less than 80 chars)

Converted from a sealed class to a ref struct to eliminate the heap allocation. Converted TestControlledRealTimeReopenThread.TestStraightForwardDemonstration() to test using ReferenceContext<T> to verify functionality. See #920.

See #920.