PG: persist divergent_priors in ondisklog

PG: persist divergent_priors in ondisklog

Consider the following logs:

a) 10'10(5'7) foo
   12'11(4'3) bar

b) 10'10(5'7) foo
   13'11(4'4) baz

When the osd with a merges primary log b, bar is deleted and
added to the missing set with need=4'3 and have=0'0.  If
the osd then dies after deleting bar, but before recovering
bar, PG::read_state() on start up will fail to re-add bar
to the missing set, and bar will be incorrect on that osd.

Now, (4'3, bar) will be added to the divergent_priors mapping
to be scanned during read_state along with the log.

Signed-off-by: Samuel Just <sam.just@inktank.com>