A queued write outlives the bundle that wrote it. After a deploy a tab can
still run the previous bundle, and an install can be rolled back, so a row
written by a newer build may be replayed by an older one that does not know
its shape.
Each row enqueue() writes now carries schemaVersion (MUTATION_SCHEMA_VERSION)
and the UI build that queued it. Both fields are optional, so rows queued
before this read as format 1 and replay as before. A flush leaves a row from a
newer format in the queue without sending, marking or dropping it, and holds
back the later writes to the same entity as it does behind a parked write, so
they keep their order until a build that knows the format replays them.
Maurice