LU-4259 osd-zfs: Add brw_stats collection and display

LU-4259 osd-zfs: Add brw_stats collection and display

Collect stats consistant with brw_stats where possible for osd-zfs.
The supported stats are: (prefixed with BRW_R and BRW_W)
 * PAGES
 * RPC_HIST
 * IO_TIME (reads only)
 * DISK_IOSIZE
 * DISCONTINOUS PAGES (only meaningful for writes, always single
buffer for reads)

Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: Icd0d703d3cf31487cdfa524613cfc550db228dba
Reviewed-on: http://review.whamcloud.com/11467
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>