Remove I2P support from netbase

I2P apparently needs 256 bits to store a fully routable address. Garlicat
requires a centralized lookup service to map the 80-bit addresses to fully
routable ones (as far as I understood), so that's not really usable in our
situation.

To support I2P routing and peer exchange for it, another solution is needed.
This will most likely imply a network protocol change, and extension of the
'addr' message.