signatures: a transfer carries one signature per writer in it

Under a policy requiring signatures, a node takes a change only once a signature
vouches for it: one of the change itself, or of a later change of the same writer. A
writer is a node's run, each start, rejoin or drop making a fresh actor, so an epoch
gathers writers for as long as it lasts, and a transfer of its whole history (a joiner
pulling the document, a delta) must carry a signature for each of them, or the joiner
waits for the rest for good.

A message's signatures are now chosen by what it carries:

* changes, up to 256 signed among them: each one's signature;
* more (a whole document pulled): each writer's latest signed change among them, found
  by the document's change metadata (`Signatures::for_changes`), vouching for the rest;
* a delta: each writer with changes in it (`get_changes_meta(from)`), its latest
  verified signature (`for_delta`).

A message carries up to 65536 (6 MiB), and as many wait at the receiver for their
changes (a delta's come after it). So a transfer leaves writers unvouched only in an
epoch with more than 65536 runs writing since it began; a remake starts the count again.
Wire version 48.

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