Improve the performance of high-level graph iterators of the C++ library

**Is your feature request related to a problem? Please describe.**
An important application case of GraphAr is to serve out-of-core graph processing scenarios. With the graph data saved as GAR files in the disk, GraphAr provides a set of reading interfaces to allow to load part of graph data into memory when needed, to conduct analytics.
Since for out-of-core graph processing, disk I/O time usually dominates the overall execution time. It is critically important that the GraphAr C++ library perform efficiently for traversing vertices/edges through high-level graph iterators.

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