Seeder does not currently distinguish between whether or not a node is
following the correct chain or not. This is a step in that direction by
making the seeder ban nodes that are do not have the latest checkpoint
in its chain. To do this, this patch allows the seeder to send GETHEADER
messages thereby activating code introduces earlier.

Depends on D4439

Edit the CTestNetParams class in chainparams.cpp to have vSeeds.emplace_back("localhost"); as the only seed.

ninja check
./bitcoind --printtoconsole --testnet -whitelist=127.0.0.1 -listen=1 -connect=0 -debug=net -datadir=<New data dir>
./bitcoin-seeder -h localhost -n blah.bitframe.org -p 8888 -m info@bitframe.org --testnet -wipeban -wipeignore

The seeder should attempt to connect to the localhost node. The node
should then be banned because its chain is not up to date and it replied
with an empty header message.

0/2 available (0 tried in 0s, 0 new, 0 active), 1 banned; 0 DNS requests, 0 db queries

The seeder should output something like the above line.

Assuming you're testnet chain is up-to-date:

./bitcoind --printtoconsole --testnet -whitelist=127.0.0.1 -listen=1 -connect=0 -debug=net
./bitcoin-seeder -h localhost -n blah.bitframe.org -p 8888 -m info@bitframe.org --testnet -wipeban -wipeignore

The seeder should attempt to connect to the localhost node. The node
should reply with a non-empty headers message and then continue to
receive an addr message from the seeder. It should then start
connecting to various other addresses received from the seeder.

0/2 available (1 tried in 2s, 0 new, 1 active), 0 banned; 0 DNS requests, 0 db queries

Seeder should output something like the above line after being successful.