Enable I2P support for DNS lookups

[chisa0a]

Related to grin#2712

Some ideas for making DNS lookups over I2P:

Wrap lookups in I2P datagrams, relay to another grin node (or two) over I2P, then make DNS call to AS.

Nodes could verify (somewhow) the certs against known seed list before making the out-of-network request at the edges.

OR

Setup in-network DNS nodes in I2P. Anyone that was willing to keep reliable service, by some metrics, could setup an eepsite.

Their b32.i2p or vanity address could be added to a list of known seeds, and seed b64 destinations could be packaged as reseed files distributed w/ source & binaries.

Maybe only keep established seeds in source, and they can flood (distribute via I2P DHT)
more short-term seeders. This would allow for a sliding scale of commitment, and maybe
propagate/decentralize seed providers.

Tunnel hop length could also be adjusted for performance or paranoia, via config option.