osd/PGLog: force write of may_include_deletes after split

osd/PGLog: force write of may_include_deletes after split

When a collection is split this needs to be persisted again. Normally
this is only persisted when the missing set is rebuilt during a new
interval when it previous did not include deletes, but during split we
keep the in-memory missing set may_include_deletes flag, but do not
rebuild the missing set.

Fixes: http://tracker.ceph.com/issues/20704
Signed-off-by: Josh Durgin <jdurgin@redhat.com>