osd: change pg_stat_t::reported from eversion_t to a pair of fields

osd: change pg_stat_t::reported from eversion_t to a pair of fields

This rarely represents an actual eversion_t as the epoch and seq values are
bumped semi-independently to ensure it is always unique.  Break it into
two separate fields to avoid confusion.

Drop now-unused and slightly curious inc() method.

Signed-off-by: Sage Weil <sage@inktank.com>