In order to filter nodes based on whether they follow chain params or
not, the seeder needs to be able to handle the HEADER
messages after sending a GETHEADER message. This diff only covers the
seeder's ability to receive and process the new type of message and ban
nodes that are not following the correct chain.
Note: the seeder cannot actually reach this new logic yet because it has no
method of requesting a HEADERS message.
Depends on D4436