add scsi_execute_async

Here are the patches Christoph requested rediffed against the current scsi-misc, plus one to add a kmemcache of scsi_io_context structs. I made the patches against scsi-misc, but re-tested them against a older kernel because the current git trees do not boot for me. This set of patches does not included the blk_queue_max_sectors() patch or the patches to convert the ULDs. I also dropped the dm-multipath hw_handler conversion patches temporarily.