remake: a backup's epoch as its snapshot names it

Review 4, appendix: a document loaded from a backup was taken to be on the largest
generation `_continuum/epochs` held as it stood, verified at its only root. In a swarm
without a policy a member's own change could plant `_continuum/epochs/99`, and a backup
then failed to load until it was removed.

The generation is now read at the document's only change without dependencies (its
snapshot writes its own entry and carries the earlier ones), as the rest of the
protocol reads epoch metadata; the genesis change writes none, so a first-epoch backup
is genesis whatever was planted. The test plants one.

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