optimize AccountIndexIterator to reuse last loaded bin-map item range

[HaoranYi]

Problem

AccountIndexIterator fetches the items from accounts map in batches, when it is configured "sorted". For each batch, it fetch the result from the underlying bin map by loading, filtering and sorting. However, when a batch only partially consumes a bin map, next batch will start loading/filtering/sorting the same bin map from the beginning again.

Imagining that a bin map contains 10K items, each batch size is 1K, then when we return 10 batches, we will be loading/filtering/sorting the bin-map for 10 times. There is a lot of waste.

We should optimize this by caching the bin-map loaded result in the iterator, so that we only load/filter/sort each bin map at most once.

Summary of Changes

Optimize AccountIndexIterator to reuse last loaded bin-map range.

Fixes #