Expose duplicate removal in the completion suggester

[mikemccand]

Lucene 6.5.0 adds an option to the NRT document suggester to efficiently remove duplicates, and it's included in the last snapshot upgrade (#23362).

This should fix #22912, but I won't have time to work on it.

[trompx]

Hello @mikemccand,

Will this option be available for the context suggester too?

I index document question with a category property so I use the context suggester to suggest question when new documents are created and filtered by category so the completion suggester would not be enough for my usecase.