sync: a message carries the signatures of its changes

A sync message carried the signatures its peer wasn't sent yet, by a
cursor per peer. Changes sent again, as after a message taken as
delivered and lost on the way, went without them: the peer was past
them. Under a policy requiring signatures they waited until the
writer's next signed change, or the next session.

A message now carries the signatures of the changes it carries
(Signatures::for_changes), whatever the peer was sent before: changes
sent again arrive vouched for as the first time. Past 256 (a message
carrying the batches of many writers), each writer's latest instead,
as a delta carries. The cursor goes, and with it resetting it after a
failed send or a new session.

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