osd/ECBackend: use an explicit backfill field on ECSubWrite

osd/ECBackend: use an explicit backfill field on ECSubWrite

Previously, we used an empty transaction to indicate when we
were sending the op to a backfill peer which needs the logs,
but can't run the transaction.  I'd like to be able to send
and empty transaction for the rollforward side effect without
it causing the peer to think it missed a backfill op, so
instead, use an explicit flag.  Compatability is handled by
interpretting an old version encoding with an empty transaction
as having the backfill field filled.

Signed-off-by: Samuel Just <sjust@redhat.com>