PG: clear out the waiting_for_missing_object list properly

We were previously not erasing empty entries, which was confusing
the rw_manager recovery locking.

Signed-off-by: Greg Farnum <greg@inktank.com>
fixup "ReplicatedPG: take recovery locks in wait_for_missing_object()"

And the subsequent
"ReplicatedPG: add a cookie to get_backfill_read() to identify requester"

Forgot to take the lock in the unfound branch, which still adds it
to the waiting_for_missing_object list.