Introducing "database reopens" into the stress test. Database will reopen after a specified number of iterations (configurable) of each thread when they will wait for the databse to reopen.

Summary: FLAGS_reopen (configurable) specifies the number of times the databse is to be reopened. FLAGS_ops_per_thread is divided into points based on that reopen field. At these points all threads come together to wait for the databse to reopen. Each thread "votes" for the database to reopen and when all have voted, the database reopens.

Test Plan: make all;./db_stress

Reviewers: dhruba, MarkCallaghan, sheki, asad, heyongqiang

Reviewed By: dhruba

Differential Revision: https://reviews.facebook.net/D6627