signatures: keep a few different signatures of a change waiting for it

A received signature of a change the document doesn't hold yet waits for
it (Pending), checked once the change comes. A delta's signatures come
before its changes, so they wait for the whole download. Kept one per
change, the last that came, a member could send a signature that
doesn't verify for a change it knows is on its way and put the honest
one aside: the writer then went unvouched for until its changes came
again with their signatures.

Pending now keeps up to three different signatures of a change
(CANDIDATES): the first that came, and the latest others. Ed25519
signatures are deterministic, so every honest copy is the same bytes and
counts once: one sent first stays, one sent after stays unless more
others follow it. Once the change comes, offer and vouch_batch check
them in turn, the first that came first, up to the first valid one,
within the bound on invalid signatures (MAX_INVALID). MAX_PENDING counts
signatures, not changes.

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