Summary:
The db_bench test was not using the specified value for the max-file-open. Fixed.

The fs readhead is switched off.

Gather statistics about the table cache and print it out at the end of the tets run.

Test Plan: Revert Plan:

Reviewers: adsharma, sc

Reviewed By: adsharma

Differential Revision: https://reviews.facebook.net/D3441