Disk Performance Model - upgrade raw disk capabilities
    I was underestimating read-ahead by half
    I was underestimating queue depth by much more

Signed-off-by: markkampe <mark.kampe@inktank.com>
Performance Models - improved FS/filestore simulation
    SimDisk: added convenience functions avgRead/Write
    SimFS: major refactoring to cleaner parameters
           now reasonably simulates XFS performance
    FileStore: refactoring for cleaner parameters
           real statistical write aggreation model
    fstest: new option for direct I/O
    filestoretest: reads are always depth=1
                   default objsize=4M

Signed-off-by: markkampe <mark.kampe@inktank.com>