[FlashGraph] simplify the code of accessing vertex index.

[zheng-da]

There is no reason to provide asynchronous access to vertex index any more. Remove all the code of accessing vertex index asynchronously. @disa-mhembere I assign this task to you. It shouldn't be very difficult. I think it can help you understand the FlashGraph internals and help you develop more applications that require to change FlashGraph.

[disa-mhembere]

ok i'm in

[disa-mhembere]

reassigning this to you @zheng-da because of the code dependencies that are involved here.