ReplicatedPG: persist write errors in the pg log

ReplicatedPG: persist write errors in the pg log

This is required to prevent re-ordering of guarded writes or deletes
in the presence of network failures and resends.

Use the existing submit_log_entries() method to initiate a repop that
only updates the pg log.

Keep the write error semantics close to the existing implementation -
if we have a buffer, return it, but do not persist the buffer for now.

Refs: http://tracker.ceph.com/issues/14468
Signed-off-by: Josh Durgin <jdurgin@redhat.com>