crimson/os/seastore, osd/PGLog: handle omap_iterate retry to avoid duplicate entries

crimson/os/seastore, osd/PGLog: handle omap_iterate retry to avoid duplicate entries

Seastore omap_iterate may retry internally on conflicts, which can
cause PGLog to process the same entries multiple times when entries
are handled directly in the iteration callback.

Introduce a conflict hook in omap_iterate so callers can reset
iteration state on retry. PGLog now buffers entries during iteration and
applies process_entry() only after a successful pass, clearing the buffer
on retry to avoid duplicates.

Signed-off-by: Myoungwon Oh <ohmyoungwon@gmail.com>