PG::can_discard_op: do discard old subopreplies

Otherwise, a sub_op_reply from a previous interval can stick around
until we either one day go active again and get rid of it or delete the
pg which is holding it on its waiting_for_active list.  While it sticks
around futily waiting for the pg to once more go active, it will cause
harmless slow request warnings.

Fixes: #9259
Backport: firefly
Signed-off-by: Samuel Just <sam.just@inktank.com>