osd/PG: fix lost unfound + delete when there are no missing objects
The default constructed pg_missing_t for a peer with no missing
objects resets may_include_deletes to false, so when missing items for
lost deletes should be added later, they are not, and the old
version of the lost object is left on the replica.
Fix this by always setting may_include_deletes upon entering the
GetMissing state, and using clear() rather than the default
constructor to create an empty missing set.
This only affects lost_unfound delete since it is the only way for a
peer to gain missing delete entries. If the peer starts out missing
some objects, its missing set sent over the wire is used instead, with
the correct setting for may_include_deletes.