* 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.
This prevents the first peer after the gateway from looking at the encrypted data received
and seeing "hey, none of the checksum blocks match the payload, they must be the gateway".
