Use forcednsseed by default prior to the fork

Summary:
Ahead of the fork, UAHF nodes are a small minority on the network
and it is possible that they are not found.
This guarantees that the ABC seeds will be asked, increasing the
chance of connecting to good peers.

Can be disabled later once fork peers are easily found without it.

Test Plan:
Repeatedly:

• stop node, remove peers.dat
• start node
• check that ABC peers are connected to via seeders after startup (using getpeerinfo RPC or GUI)

A newly started node should be able to connect to ABC peers very
rapidly.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D360