Add db_bench option --report_file_operations

Summary: Add db_bench option --report_file_operations

Test Plan:
./db_bench --report_file_operations
Observe outputs on # of file operations

Reviewers: ljin, MarkCallaghan, sdong

Reviewed By: sdong

Subscribers: yhchiang, rven, igor, dhruba

Differential Revision: https://reviews.facebook.net/D27945