Peer library: remote connection to a transactor, tx-report handling,
local database values, sync, and the segment cache
(see docs/architecture.md and docs/design/protocol.md).
A [Connection] subscribes to the transactor's tx-report stream and
folds every report into an immutable [Db] value locally; queries never
block on the transactor. On disconnect it reconnects and resubscribes
from its basis, and the server backfills the gap from the durable log.