Add iterate_devices flashcache function. Up the higher bound on dirty thresh to 95% (from 90%).

Summary:
1) Add iterate_devices flashcache function. Needed to make flashcache work with 4KB sectored
"disk" devices.
2) Up the higher bound on dirty thresh to 95% (from 90%).
Thanks to Kumar Sundararajan (facebook) for help with this change !

Test Plan:

Reviewers:

CC:

Task ID: #

Blame Rev: