Quinn: Remote memory exhaustion in quinn-proto from unbounded out-of-order stream reassembly

Summary

The Assembler component that assembles unordered stream fragments into consecutive chunks of the stream incurs some overhead for non-contiguous fragments. Readers that read from a RecvStream in order (through an AsyncRead impl for example) will be sensitive to peers that send fragments while leaving out early parts of the stream, and in particular, fragments with many gaps (because these cannot be defragmented). In such a scenario, the receiving connection suffers from high buffer overhead, enabling memory exhaustion.