osd: read kb stats not tracked?

In read cases track stats in PG::unstable_stats
Include unstable_stats in write_info() and publish_stats_to_osd()
For now this information may not get persisted

fixes: #2209

Signed-off-by: David Zafman <david.zafman@inktank.com>
Set num_rd, num_wr_kb and num_wr in various places that needed it

Signed-off-by: David Zafman <david.zafman@inktank.com>