document file: write as a fresh actor once the file is marked closed

With --keep-actor, a node on a file whose last record is Closed writes on
as the actor the header names, which takes the file holding every change
of that actor. At a clean stop, the node's own workers go on writing
until it exits: its alive stamp in the member list, votes, a
coordinator's commit. One committed after Closed was appended, and sent
by the still-running sync, would be a change of that actor the file
lacks, and the next start would make another change with its number.

swarm::document_file::close now takes the document's write lock, marks
the file at the heads it sees there, and sets a fresh actor before
letting go: what the node writes after is no change of the marked actor.
A record the file worker appends after Closed (such a write, flushed)
leaves the file unmarked, which starts a fresh actor next time.

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