Add backpressure to simplex outgoing sync sessions
SimplexOutgoingSession reads acks and messages 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 acks and messages being queued. This uses an unbounded amount of memory and increases the risk of acks or messages 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.
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