* added more inbound tests
* made the tunnel preprocessing header more clear and included better fragmentation support
(still left: tests for outbound tunnel processing, structures and jobs to integrate with the router,
remove that full SHA256 from each and every I2NPMessage or put a smaller one at the
transport layer, and all the rest of the tunnel pooling/building stuff)
tunnel ID they listen on and make sure the previous peer doesn't change over time. The
worst that a hostile peer could do is create a multiplicative work factor - they send N
messages, causing N*#hops in the loop of bandwidth usage. This is identical to the hostile
peer simply building a pair of tunnels and sending N messages through them.
also added some discussion about the tradeoffs and variations wrt fixed size tunnel messages.
