network: Buffer partial reads for cancellation safety

[joeykraut]

Purpose

This PR adds buffers for partial reads in the network layer of the MpcFabric. This is necessary because tokio::select may cancel a read future, which drops the underlying partially read buffer of the next message without resetting the quic stream's cursor.

This means the next time a read future is created, it will begin from the middle of a message in the quic stream, causing an invalid read or blocking on data that will never be available.

The fix is to buffer partial reads in the QuicTwoPartyNet instance, which is maintained across cancelled futures.

Testing

• Unit and integration tests pass
• Replicated the issue with a large circuit in renegade::circuits and verified that this issue was fixed

[akirillo]

lgtm, left a small q / suggestion