Code Cleanup + Optimizations

[CascadingRadium]

• Cleanup up unused code paths since the merging of MB-59102: Merging kNN hits from a distributed index with exact search hits #1936 changed the overall approach, resulting in a lot of dead code.
• Optimized the KNN collector to not update the allHits map for every hit and instead just use its internal heaps when the Final() is called.
• Typo Fix
• Optimized the OptimizeVR's Finish() method to concurrently search across segments.

Results of Optimization

Dataset - SIFT 1 Million dataset
Index - 1 partition index

Before this commit - Query Execution time - 2.25 seconds
With this commit - Query Execution time - 1.70 seconds
Percentage Reduction - 24.4%

[abhinavdangeti]

@CascadingRadium let's have you walk the team through this real quick on tomorrow's call. We can move ahead with review and merging after.