Performance: attempt to optimize the ArrayHitCounter by maintaining some state while updating the counter

[alexklibisz]

Related Issue

#160, #611

Changes

I had two ideas to speed up the ArrayHitCouner here, but they don't seem to be panning out in the benchmarks:

1. Maintain the histogram that's used to determine the kthGreatest value while updating the counter, i.e., each time we encounter a document.
2. Maintain the minimum and maximum document ID for a given count while updating the counter, so that we can limit the DocIdSetIterator to start and end at the min/max doc IDs of the documents with counts that qualify to be a candidate. I removed this idea in 2624988.

Testing and Validation

How was it validated?