Adds experimental session sync (`@earendil-works/pi-durable/sync`).
Devices upload and pull a session's commits through a remote, reading
them from the Chord store's outbox and applying them with `applyBatch`.
If two devices continue the same session, the one that uploads second
forks to a new session.
I took out the extra sequence indexes from the earlier version of this
branch, since the Chord outbox already persists every commit, so this no
longer touches storage.
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Co-authored-by: Armin Ronacher <armin.ronacher@active-4.com>
