Summary:
Add async_io Read option in db_bench

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9735

Test Plan:
./db_bench -use_existing_db=true
-db=/tmp/prefix_scan_prefetch_main -benchmarks="seekrandom" -key_size=32
-value_size=512 -num=5000000 -use_direct_reads=true -seek_nexts=327680
-duration=120 -ops_between_duration_checks=1 -async_io=1

Reviewed By: riversand963

Differential Revision: D35058482

Pulled By: akankshamahajan15

fbshipit-source-id: 1522b638c79f6d85bb7408c67f6ab76dbabeeee7