osd: allow multiple txns to be queued via backend

Signed-off-by: Sage Weil <sage@redhat.com>
osd: avoid txn append in ReplicatedBackend::submit_transaction

Signed-off-by: Sage Weil <sage@redhat.com>
osd: avoid txn append in ReplicatedBackend::sub_op_modify

Signed-off-by: Sage Weil <sage@redhat.com>
wip