HAMMER VFS - Implement async I/O for double-buffer strategy case

HAMMER VFS - Implement async I/O for double-buffer strategy case

* When vfs.hammer.double_buffer is enabled the HAMMER strategy code
  was running synchronously.  This creates numerous problems including
  extra stalls when read-ahead is issued.

* Use the new breadcb() function to allow nominal double_buffer strategy
  operations to run asynchronously.  Essentially the original buffer and
  CRC is recorded in the device bio and the copyback is made in the
  callback.

* This improves performance vfs.hammer.double_buffer is enabled.