Add cancellation support to IndexSearcher

[paulirwin]

Is there an existing issue for this?

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Is your feature request related to a problem? Please describe the problem.

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Describe the solution you'd like

From @NightOwl888:

What I was wondering is whether it makes sense to be able to cancel an in progress search (and whether it would function as expected). We are using a .NET TaskScheduler here and CancellationToken is part of the tool set. This isn't something that exists in Lucene 4.8.0, but I think it would be useful to have in .NET if it can be implemented reliably. That is, if there isn't some other way to cancel a search that I am unaware of.

Additional context

Context: #914 (comment)