add bookshelf stress tests

The bookshelf bench stress tests are intended to put bookshelf under
sufficient load so that we can attch to the vm and use debugging
tools and techniques to monitor the performance and health of the
bookshelf system.
Revert "Convert file IO to raw mode"

This reverts commit 12f2c91283c7387365dc54b7e4b16ec1236e06e5.

Reverting this commit fixes the process consumption behavior that we've
been seeing with the latest builds of bookshelf under heavy load. The
stress tests in the `stest` directory can be used to confirm the
behavior.