Revert "Revert "Convert file IO to raw mode""

This reverts commit d6b935d34c4210d9e9450cb0749716e4e4ff44ae.

This restores the usage of raw IO mode. We had seen earlier failures
of bookshelf under load when running in raw mode, and had
(incorrectly) blamed this patch for the problem.

Further research shows that the load failures were due to logging
backing up. Enabling raw mode appears to have sped up the rest of the
system, causing the logging problem to present sooner.
Increase async worker thread pool size to 10

Increasing the number of async worker threads from the defaut of 0
increases the performanc of IO-heavy apps like bookshelf.

Load testing different configurations of +A resulted in a large
performance increase from 0 to 8 workers, and no appreciable
preformance increase form 8 to 16. The value 10 was chosen as
it is the default in Erlang R16B.
Merge branch 'ma/enable-raw-file-io'
bookshelf 1.1.3