document file: a clean stop marked, and `--keep-actor` writes on as the same actor

A node writes as a fresh Automerge actor at each start, so no change number of an actor
comes twice: one sent before a crash but not yet appended to the document file would
otherwise be made again, differently. An epoch thus gathers a writer per start, and
every transfer of its history carries a signature per writer.

The document file now says when it holds everything the node wrote:

* at a clean stop, after the last append, an empty `Closed` record, only where the
  file is up to the document's heads (ready, not rejoining);
* as a node starts on the file, before anything is written, an empty `Opened` record,
  so a crash from then on leaves a file without the mark;
* the header names the actor the node writes with (`current`).

With `--keep-actor` (off by default; needs `--document-file`), a node starting on a
file that ends with `Closed` writes on as that actor. A crash, a drop, a switch or a
rejoin still starts a fresh one, so an epoch's writers come to one per node plus one
per such event. lifecycle.md says so.

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