OSD fails to start with error assert(values.size() == 2)

Remove redundant setting of dirty_info
Fix PG::upgrade() to conform to programming style
Set dirty_big_info in PG::write_info for upgrades

BUG: #4562

Signed-off-by: David Zafman <david.zafman@inktank.com>
Reviewed-by: Sam Just <sam.just@inktank.com>