[Stgt-devel] AIO code is removed

[FUJITA Tomonori]

The iSCSI driver used AIO, however it has been painful:

- not rock solid
- no unified event notification
- no page-cache AIO support
- no AIO FSYNC support

The first issue is critical and the third and forth issues hurt
scalability and performance, though we can live with the third one.

I added new backing store code, bs_sync, which uses pthread and
synchronous I/Os though I don't like to play with pthread.

Now the iSCSI driver uses bs_sync code by default. Please test it.