state: a document swap keeps the signatures of the changes it keeps

Review 2.2: swap_document forgot every signature. After dropping
changes taken unjudged (or a rejoin into the same epoch), the node
could vouch for none of the changes it kept: under signatures required,
those building on an actor waited for that actor's next signature, and
for an actor that stopped writing, forever.

Signatures::kept_for keeps the signatures of the changes the new
document holds, checks each actor's latest again, and keeps those
waiting for their changes. This node's replayed changes are signed at
once, as before.

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