Handle incoming extend2/extended2 fragmented requests/replies. (prop249)
As part of prop249, we need to handle large create cells when they arrive fragmented across multiple extend cells (and similarly for created cells fragmented across multiple extended cells).
Important notes:
- The proposal says that there must be no intervening cells on the same circuit. We should enforce this and test it.
- We should probably use a buf_t object to record these handshakes as they are being assembled.
- We should count these handshakes against the memory usage of a circuit and age of the oldest queued data, so that they will participate correctly in the OOM system.