signatures: the latest unsigned change of each own actor signed, at start and after a swap

Review 4, F2 (the honest case) and claim 2: a node signed at start its own changes at
the heads that nothing vouched for (1216), and after a swap only its current actor's
latest. An own change left unsigned can sit below the heads: a drop (`drop_taken`)
takes away a signed change and keeps an earlier unsigned one, and the node writes on
under a fresh actor, building on it; or a node's change is built on by another's
concurrently with the signed one the drop took. Nothing signed it again, and under a
policy requiring signatures every node pulling it (a joiner) waited for good, and for
everything built on it.

Now the latest change of each of the node's actors (`RemakeState::own_actors`, the
current one among them) that no signature vouches for is signed
(`Signatures::sign_own_actors`), at start and whenever the document is swapped (a drop,
a switch, a rejoin), the old `sign_own_heads` and `sign_own_latest` going. One
signature per actor vouches for all its changes before it. The case of a member whose
signed change the rules refuse, vouching once for its earlier ones, is
postponed (future-work.md, next).

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