Add backpressure to outgoing duplex sync sessions
DuplexOutgoingSession reads records from the database as quickly as possible and queues them for transmission. If the DB is faster than the transport, this will result in all sendable records being queued. This uses an unbounded amount of memory and increases the risk of records being lost before they're transmitted, leading to unnecessary retransmissions.
Add a backpressure mechanism that limits the amount of queued data and delays DB reads when the queue is full.
This will be a bit more complex than #1319 because DuplexOutgoingSession can start DB queries in response to events.
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