BREAKING: Add cancellation support to IndexSearcher, #922

[paulirwin]

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Adds basic cancellation support to IndexSearcher.

Fixes #922

Description

This adds optional CancellationToken parameters to all IndexSearcher.Search and SearchAfter methods, which is a breaking change to the API.

If an executor is provided in the constructor for multithreaded search, then the provided CancellationToken is passed to the Task.Wait(CancellationToken) method. If an executor is not provided, then it will call ThrowIfCancellationRequested() in the synchronous case at the evaluation of each leaf reader context.

This will not help cancellation for i.e. single-leaf reader contexts that might take a while to complete (that would require a much larger and messier change to add CancellationToken support to Scorer and ICollector, and benchmarking to ensure we don't hurt performance by doing so), but should at least help with many scenarios that require cancellation.

I wanted to publish this draft PR for feedback; please let me know if you like this direction or have concerns.